Code
P1408
ALFA ROMEO
P — Powertrain
Coolant heating circuit fault | Heater circuit fault
Views:
UK: 4
EN: 6
RU: 1
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
BUICK
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 18
EN: 27
RU: 25
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
CADILLAC
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 19
EN: 25
RU: 14
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
CHEVROLET
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 18
EN: 24
RU: 14
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
CHRYSLER
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 20
EN: 25
RU: 15
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
FIAT
P — Powertrain
Coolant heating circuit fault | Heater circuit fault
Views:
UK: 4
EN: 5
RU: 2
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
FORD
P — Powertrain
EGR Flow Out of Self-Test Range
Views:
UK: 17
EN: 26
RU: 18
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
GEO
P — Powertrain
MAP Sensor Circuit
Views:
UK: 16
EN: 28
RU: 19
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
GM
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 15
EN: 25
RU: 15
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
GMC
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 17
EN: 25
RU: 18
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
HUMMER
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 6
EN: 15
RU: 5
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
JAGUAR
P — Powertrain
Exhaust Gas Recirculation Thermister
Views:
UK: 22
EN: 26
RU: 19
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
LAND ROVER
P — Powertrain
Exhaust gas recirculation flow out of autotest range
Views:
UK: 4
EN: 6
RU: 2
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
Workshop Manuals
Repair manuals for LAND ROVER
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Land Rover Defender 300Tdi — Workshop Manual (1996 model year)
Workshop Manual199,00 UAH
Land Rover Defender Workshop Manual Supplement & Body Repair Manual (1999 & 2002 MY)
Workshop Manual199,00 UAH
Land Rover Range Rover — Electrical Library (LRL 0453ENG, 2002)
Workshop Manual199,00 UAH
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Code
P1408
LINCOLN
P — Powertrain
EGR Flow Out of Self-Test Range
Views:
UK: 20
EN: 24
RU: 22
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
MAZDA
P — Powertrain
EGR System Flow Out Of Key On Engine Running Self Test Range
Views:
UK: 18
EN: 24
RU: 14
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
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Code
P1408
MERCURY
P — Powertrain
EGR Flow Out of Self-Test Range
Views:
UK: 22
EN: 25
RU: 13
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
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Code
P1408
OLDSMOBILE
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 23
EN: 23
RU: 15
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
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Code
P1408
Other
P — Powertrain
EGR Flow Out Of Self Test Range
Views:
UK: 19
EN: 28
RU: 17
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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Code
P1408
PLYMOUTH
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 21
EN: 27
RU: 15
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
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Code
P1408
PONTIAC
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 17
EN: 24
RU: 15
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
Your experience will help others
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Was this AI description helpful?
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👍 Like
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Send to email
Code
P1408
SATURN
P — Powertrain
Intake Manifold Pressure Sensor Circuit
Views:
UK: 21
EN: 27
RU: 17
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
Your experience will help others
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👍 Like
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Send to email
Code
P1408
SUZUKI
P — Powertrain
Maniforld Differential Pressure Sensor Circuit Fault
Views:
UK: 18
EN: 21
RU: 14
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
Similar codes
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Was this AI description helpful?
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Send to email
Code
P1408
VOLKSWAGEN
P — Powertrain
EGR Temperature Sensor Signal Too High
Views:
UK: 17
EN: 27
RU: 19
AI status
Completed
Completed
100%
Causes
- Blown fuse or faulty relay in heater circuit
- Open or shorted wiring between ECM and heater element/relay
- Corroded or loose connector at heater element or relay
- Failed heater element (high resistance or open)
- Control module (ECU) driver output fault
- Low coolant level or thermostat stuck open preventing expected temperature conditions
Symptoms
- Diminished or no coolant heating (delayed cabin warm-up or poor cold start warm-up)
- DTC P1408 stored; possible MIL illumination
- Radiator or heater hoses remain cold longer than expected
- Intermittent operation of the heater
- Possible electrical smell or melted connector in severe cases
What to check
- Read and record freeze-frame and full DTC data (store/clear history).
- Visual inspection of wiring, connectors, fuse(s) and relay(s) for the coolant/heater circuit.
- Check battery voltage and main power supply to heater circuit fuse/relay.
- Measure continuity and resistance of heater element to ground (with coolant drained if required by service manual).
- Back-probe heater connector and check for commanded voltage/duty cycle while performing an active test with scan tool.
- Check for parasitic short to ground or short to battery on heater circuit wiring.
Signal parameters
- Supply voltage at heater fuse/relay: nominal battery voltage ~12–14 V (engine running).
- Activation signal from ECU: switched 12 V or PWM duty cycle depending on system (0–100% duty).
- Heater element resistance: low ohm range (typical values vary by model—check service manual); open or very high resistance indicates failure.
- Circuit current draw when energized: several amps (measure with clamp meter) — abnormally high indicates short, abnormally low or zero indicates open/high resistance.
- No voltage at connector with heater commanded indicates open supply or relay fault; presence of voltage but no current can indicate open element.
Diagnostic algorithm
- Retrieve freeze-frame and confirm current/completed status of P1408. Note conditions when code set (ambient temp, engine temp, key state).
- Perform a careful visual inspection of wiring, connectors, fuse(s) and relay(s) for damage, corrosion, or overheating. Replace or repair as needed.
- Verify power and ground to the heater circuit: check fuse continuity and relay operation. Replace suspect fuse/relay before further testing.
- With ignition off, disconnect heater element connector and measure resistance of the heater element. Compare to factory spec (open/infinite indicates failed element).
- Back-probe the heater connector and command heater ON via a factory scan tool or active test. Confirm presence of commanded voltage at the connector and measure current draw.
- If commanded voltage present but element not energizing, suspect open element or poor connector/ground. If no commanded voltage, test ECU output driver for continuity to ground or short to voltage per service manual procedures.
- Check wiring harness continuity between ECU/relay and heater element; repair any open/short circuits. Check for insulation damage near moving parts or heat sources.
- Verify engine ground locations related to heater and ECU; clean and retighten grounds as required.
- After repairs, clear codes, perform heater activation and run vehicle to normal operating conditions. Confirm code does not return and that heater functions correctly.
- If tests indicate ECU driver failure, confirm with service manual diagnostics before replacing the control module.
Likely causes
- Failed heater element or high resistance in heater
- Supply fuse or relay failed
- Connector corrosion or damaged wiring harness near engine or heater
- ECU output transistor or driver fault
- Ground connection degraded or missing
Fault status
Status
ECU detected a fault in the coolant/heater circuit (open, short, high resistance or intermittent). Diagnostic testing of fuse, relay, wiring, heater element and ECU output required.
Repair difficulty: Medium
Diagnostic time: 0.8-2.5 hours
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