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Home / DTC / P0128 — Defective Engine Thermostat, Defective Engine Coolant Temperature Sensor, Defective Intake Air Temperature Sensor, Defective Cooling System, Low Engine Coolant, Dirty Engine Coolant causing incorrect Coolant Temperature Sensor readings, Defective/always running Engine Cooling fan(s)

P0128 — Defective Engine Thermostat, Defective Engine Coolant Temperature Sensor, Defective Intake Air Temperature Sensor, Defective Cooling System, Low Engine Coolant, Dirty Engine Coolant causing incorrect Coolant Temperature Sensor readings, Defective/always running Engine Cooling fan(s)

Detailed page for trouble code P0128.

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Code

P0128

AUDI P — Powertrain

Defective Engine Thermostat, Defective Engine Coolant Temperature Sensor, Defective Intake Air Temperature Sensor, Defective Cooling System, Low Engine Coolant, Dirty Engine Coolant causing incorrect Coolant Temperature Sensor readings, Defective/always running Engine Cooling fan(s)

Brand: AUDI
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Thermostat stuck open or opening too early
  • Defective Engine Coolant Temperature (ECT) sensor or connector/wiring fault
  • Low engine coolant level or air trapped in cooling system
  • Restricted coolant flow (water pump, blocked passages, collapsed hose)
  • Contaminated or degraded coolant affecting heat transfer
  • Intake Air Temperature (IAT) sensor fault (causing incorrect warm-up strategy)

Symptoms

  • Check Engine Light (MIL) illuminated and P0128 stored
  • Engine takes a long time to reach normal operating temperature
  • Cabin heater produces weak or cold airflow
  • Reduced fuel economy and possibly richer-than-normal running during warm-up
  • Temperature gauge reads lower than normal or fluctuates
  • Cooling fans running more often than expected

What to check

  • Scan tool: read live coolant temperature (ECT) and IAT, freeze-frame data and readiness status
  • Inspect coolant level and reservoir for proper fill and contamination
  • Visually inspect wiring and connectors at ECT and IAT sensors for corrosion or damage
  • Confirm cooling fan operation and verify fan is not running continuously at low temp
  • Check for recent coolant service or air in system (signs of bubbles, recent repairs)
  • Compare heater output at dash (heater core warm-up) to coolant temperature readings

Signal parameters

  • Typical normal coolant operating temperature: 80–105 °C (176–221 °F) depending on model and thermostat
  • Thermostat opening (typical) ~85–95 °C (185–203 °F) — model dependent
  • Fans typically command ON around 95–105 °C (203–221 °F) (varies by model/engine)
  • Expected warm-up: coolant should rise steadily from ambient and reach ~70–90 °C within several minutes at idle or short drive — if it stays near ambient, suspect stuck-open thermostat or sensor fault
  • ECT sensor resistance/voltage: compare to manufacturer table with scan tool or multimeter (expected voltage decreases as temp rises; typical 0.5–4.5 V/OTC varies)

Diagnostic algorithm

  1. Connect a calibrated scan tool, read freeze-frame and live data: record coolant temp at cold start and after 5–10 minutes of idle/driving.
  2. Verify ambient temperature and compare expected warm-up time to observed data.
  3. Inspect coolant level, top up if low, and bleed air from system if required. Re-test warm-up behavior.
  4. Check ECT sensor: inspect connector and wiring for corrosion, measure sensor resistance/voltage and compare to spec. Wiggle test wiring while monitoring live data for intermittent faults.
  5. Confirm IAT sensor readings are plausible; replace or repair if out of range and affecting warm-up strategy.
  6. Verify thermostat operation: with engine cold, start and monitor temperature increase and feel upper and lower radiator hoses (use caution). A stuck-open thermostat shows little temperature rise/low delta between hose and engine block.
  7. If thermostat suspect, perform a controlled check by removing thermostat (if accessible) or replacing with correct OEM thermostat and retest.
  8. Verify cooling fan control: ensure fans are not commanded ON at low temps due to failed relay/module; check fan relay, temperature switch or control module.
  9. Inspect for restricted coolant flow: check water pump operation, heater core/hoses and radiator for blockages.
  10. After repairs, clear codes, perform a warm-up/drive cycle and confirm code does not return and readiness monitors set.

Likely causes

  • Thermostat stuck open (most common on Audi with long warm-up)
  • Faulty ECT sensor or intermittent connector/circuit short to ground
  • Low coolant level or air pocket after recent service
  • Cooling fan relay/stuck fan causing overcooling
  • Dirty or degraded coolant reducing heat transfer efficiency
  • IAT sensor fault feeding incorrect intake temperature to the ECU (less common)

Fault status

⚠️ Status
P0128 — Engine coolant temperature below thermostat regulating temperature; warm-up not achieved within expected time.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0125 P0117 P0118 P0112 P0113 P0480
Repair manuals

Manual library for AUDI

1,243

Browse 1,243 AUDI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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Code

P0128

Generic P — Powertrain

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Brand: Generic
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Thermostat stuck open or opening too early
  • Defective Engine Coolant Temperature (ECT) sensor or connector/wiring fault
  • Low engine coolant level or air trapped in cooling system
  • Restricted coolant flow (water pump, blocked passages, collapsed hose)
  • Contaminated or degraded coolant affecting heat transfer
  • Intake Air Temperature (IAT) sensor fault (causing incorrect warm-up strategy)

Symptoms

  • Check Engine Light (MIL) illuminated and P0128 stored
  • Engine takes a long time to reach normal operating temperature
  • Cabin heater produces weak or cold airflow
  • Reduced fuel economy and possibly richer-than-normal running during warm-up
  • Temperature gauge reads lower than normal or fluctuates
  • Cooling fans running more often than expected

What to check

  • Scan tool: read live coolant temperature (ECT) and IAT, freeze-frame data and readiness status
  • Inspect coolant level and reservoir for proper fill and contamination
  • Visually inspect wiring and connectors at ECT and IAT sensors for corrosion or damage
  • Confirm cooling fan operation and verify fan is not running continuously at low temp
  • Check for recent coolant service or air in system (signs of bubbles, recent repairs)
  • Compare heater output at dash (heater core warm-up) to coolant temperature readings

Signal parameters

  • Typical normal coolant operating temperature: 80–105 °C (176–221 °F) depending on model and thermostat
  • Thermostat opening (typical) ~85–95 °C (185–203 °F) — model dependent
  • Fans typically command ON around 95–105 °C (203–221 °F) (varies by model/engine)
  • Expected warm-up: coolant should rise steadily from ambient and reach ~70–90 °C within several minutes at idle or short drive — if it stays near ambient, suspect stuck-open thermostat or sensor fault
  • ECT sensor resistance/voltage: compare to manufacturer table with scan tool or multimeter (expected voltage decreases as temp rises; typical 0.5–4.5 V/OTC varies)

Diagnostic algorithm

  1. Connect a calibrated scan tool, read freeze-frame and live data: record coolant temp at cold start and after 5–10 minutes of idle/driving.
  2. Verify ambient temperature and compare expected warm-up time to observed data.
  3. Inspect coolant level, top up if low, and bleed air from system if required. Re-test warm-up behavior.
  4. Check ECT sensor: inspect connector and wiring for corrosion, measure sensor resistance/voltage and compare to spec. Wiggle test wiring while monitoring live data for intermittent faults.
  5. Confirm IAT sensor readings are plausible; replace or repair if out of range and affecting warm-up strategy.
  6. Verify thermostat operation: with engine cold, start and monitor temperature increase and feel upper and lower radiator hoses (use caution). A stuck-open thermostat shows little temperature rise/low delta between hose and engine block.
  7. If thermostat suspect, perform a controlled check by removing thermostat (if accessible) or replacing with correct OEM thermostat and retest.
  8. Verify cooling fan control: ensure fans are not commanded ON at low temps due to failed relay/module; check fan relay, temperature switch or control module.
  9. Inspect for restricted coolant flow: check water pump operation, heater core/hoses and radiator for blockages.
  10. After repairs, clear codes, perform a warm-up/drive cycle and confirm code does not return and readiness monitors set.

Likely causes

  • Thermostat stuck open (most common on Audi with long warm-up)
  • Faulty ECT sensor or intermittent connector/circuit short to ground
  • Low coolant level or air pocket after recent service
  • Cooling fan relay/stuck fan causing overcooling
  • Dirty or degraded coolant reducing heat transfer efficiency
  • IAT sensor fault feeding incorrect intake temperature to the ECU (less common)

Fault status

⚠️ Status
P0128 — Engine coolant temperature below thermostat regulating temperature; warm-up not achieved within expected time.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0125 P0117 P0118 P0112 P0113 P0480
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Code

P0128

HUMMER P — Powertrain

Coolant Thermostat Below Thermostat Regulating Temperature

Brand: HUMMER
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Thermostat stuck open or opening too early
  • Defective Engine Coolant Temperature (ECT) sensor or connector/wiring fault
  • Low engine coolant level or air trapped in cooling system
  • Restricted coolant flow (water pump, blocked passages, collapsed hose)
  • Contaminated or degraded coolant affecting heat transfer
  • Intake Air Temperature (IAT) sensor fault (causing incorrect warm-up strategy)

Symptoms

  • Check Engine Light (MIL) illuminated and P0128 stored
  • Engine takes a long time to reach normal operating temperature
  • Cabin heater produces weak or cold airflow
  • Reduced fuel economy and possibly richer-than-normal running during warm-up
  • Temperature gauge reads lower than normal or fluctuates
  • Cooling fans running more often than expected

What to check

  • Scan tool: read live coolant temperature (ECT) and IAT, freeze-frame data and readiness status
  • Inspect coolant level and reservoir for proper fill and contamination
  • Visually inspect wiring and connectors at ECT and IAT sensors for corrosion or damage
  • Confirm cooling fan operation and verify fan is not running continuously at low temp
  • Check for recent coolant service or air in system (signs of bubbles, recent repairs)
  • Compare heater output at dash (heater core warm-up) to coolant temperature readings

Signal parameters

  • Typical normal coolant operating temperature: 80–105 °C (176–221 °F) depending on model and thermostat
  • Thermostat opening (typical) ~85–95 °C (185–203 °F) — model dependent
  • Fans typically command ON around 95–105 °C (203–221 °F) (varies by model/engine)
  • Expected warm-up: coolant should rise steadily from ambient and reach ~70–90 °C within several minutes at idle or short drive — if it stays near ambient, suspect stuck-open thermostat or sensor fault
  • ECT sensor resistance/voltage: compare to manufacturer table with scan tool or multimeter (expected voltage decreases as temp rises; typical 0.5–4.5 V/OTC varies)

Diagnostic algorithm

  1. Connect a calibrated scan tool, read freeze-frame and live data: record coolant temp at cold start and after 5–10 minutes of idle/driving.
  2. Verify ambient temperature and compare expected warm-up time to observed data.
  3. Inspect coolant level, top up if low, and bleed air from system if required. Re-test warm-up behavior.
  4. Check ECT sensor: inspect connector and wiring for corrosion, measure sensor resistance/voltage and compare to spec. Wiggle test wiring while monitoring live data for intermittent faults.
  5. Confirm IAT sensor readings are plausible; replace or repair if out of range and affecting warm-up strategy.
  6. Verify thermostat operation: with engine cold, start and monitor temperature increase and feel upper and lower radiator hoses (use caution). A stuck-open thermostat shows little temperature rise/low delta between hose and engine block.
  7. If thermostat suspect, perform a controlled check by removing thermostat (if accessible) or replacing with correct OEM thermostat and retest.
  8. Verify cooling fan control: ensure fans are not commanded ON at low temps due to failed relay/module; check fan relay, temperature switch or control module.
  9. Inspect for restricted coolant flow: check water pump operation, heater core/hoses and radiator for blockages.
  10. After repairs, clear codes, perform a warm-up/drive cycle and confirm code does not return and readiness monitors set.

Likely causes

  • Thermostat stuck open (most common on Audi with long warm-up)
  • Faulty ECT sensor or intermittent connector/circuit short to ground
  • Low coolant level or air pocket after recent service
  • Cooling fan relay/stuck fan causing overcooling
  • Dirty or degraded coolant reducing heat transfer efficiency
  • IAT sensor fault feeding incorrect intake temperature to the ECU (less common)

Fault status

⚠️ Status
P0128 — Engine coolant temperature below thermostat regulating temperature; warm-up not achieved within expected time.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0125 P0117 P0118 P0112 P0113 P0480
Repair manuals

Manual library for HUMMER

138

Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HUMMER

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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

P0128

LAND ROVER P — Powertrain

Range / performance of thermostat

Brand: LAND ROVER
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Thermostat stuck open or opening too early
  • Defective Engine Coolant Temperature (ECT) sensor or connector/wiring fault
  • Low engine coolant level or air trapped in cooling system
  • Restricted coolant flow (water pump, blocked passages, collapsed hose)
  • Contaminated or degraded coolant affecting heat transfer
  • Intake Air Temperature (IAT) sensor fault (causing incorrect warm-up strategy)

Symptoms

  • Check Engine Light (MIL) illuminated and P0128 stored
  • Engine takes a long time to reach normal operating temperature
  • Cabin heater produces weak or cold airflow
  • Reduced fuel economy and possibly richer-than-normal running during warm-up
  • Temperature gauge reads lower than normal or fluctuates
  • Cooling fans running more often than expected

What to check

  • Scan tool: read live coolant temperature (ECT) and IAT, freeze-frame data and readiness status
  • Inspect coolant level and reservoir for proper fill and contamination
  • Visually inspect wiring and connectors at ECT and IAT sensors for corrosion or damage
  • Confirm cooling fan operation and verify fan is not running continuously at low temp
  • Check for recent coolant service or air in system (signs of bubbles, recent repairs)
  • Compare heater output at dash (heater core warm-up) to coolant temperature readings

Signal parameters

  • Typical normal coolant operating temperature: 80–105 °C (176–221 °F) depending on model and thermostat
  • Thermostat opening (typical) ~85–95 °C (185–203 °F) — model dependent
  • Fans typically command ON around 95–105 °C (203–221 °F) (varies by model/engine)
  • Expected warm-up: coolant should rise steadily from ambient and reach ~70–90 °C within several minutes at idle or short drive — if it stays near ambient, suspect stuck-open thermostat or sensor fault
  • ECT sensor resistance/voltage: compare to manufacturer table with scan tool or multimeter (expected voltage decreases as temp rises; typical 0.5–4.5 V/OTC varies)

Diagnostic algorithm

  1. Connect a calibrated scan tool, read freeze-frame and live data: record coolant temp at cold start and after 5–10 minutes of idle/driving.
  2. Verify ambient temperature and compare expected warm-up time to observed data.
  3. Inspect coolant level, top up if low, and bleed air from system if required. Re-test warm-up behavior.
  4. Check ECT sensor: inspect connector and wiring for corrosion, measure sensor resistance/voltage and compare to spec. Wiggle test wiring while monitoring live data for intermittent faults.
  5. Confirm IAT sensor readings are plausible; replace or repair if out of range and affecting warm-up strategy.
  6. Verify thermostat operation: with engine cold, start and monitor temperature increase and feel upper and lower radiator hoses (use caution). A stuck-open thermostat shows little temperature rise/low delta between hose and engine block.
  7. If thermostat suspect, perform a controlled check by removing thermostat (if accessible) or replacing with correct OEM thermostat and retest.
  8. Verify cooling fan control: ensure fans are not commanded ON at low temps due to failed relay/module; check fan relay, temperature switch or control module.
  9. Inspect for restricted coolant flow: check water pump operation, heater core/hoses and radiator for blockages.
  10. After repairs, clear codes, perform a warm-up/drive cycle and confirm code does not return and readiness monitors set.

Likely causes

  • Thermostat stuck open (most common on Audi with long warm-up)
  • Faulty ECT sensor or intermittent connector/circuit short to ground
  • Low coolant level or air pocket after recent service
  • Cooling fan relay/stuck fan causing overcooling
  • Dirty or degraded coolant reducing heat transfer efficiency
  • IAT sensor fault feeding incorrect intake temperature to the ECU (less common)

Fault status

⚠️ Status
P0128 — Engine coolant temperature below thermostat regulating temperature; warm-up not achieved within expected time.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0125 P0117 P0118 P0112 P0113 P0480
320

Browse 320 LAND ROVER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

LAND ROVER

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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

P0128

MITSUBISHI P — Powertrain

Coolant thermostat

Brand: MITSUBISHI
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Thermostat stuck open or opening too early
  • Defective Engine Coolant Temperature (ECT) sensor or connector/wiring fault
  • Low engine coolant level or air trapped in cooling system
  • Restricted coolant flow (water pump, blocked passages, collapsed hose)
  • Contaminated or degraded coolant affecting heat transfer
  • Intake Air Temperature (IAT) sensor fault (causing incorrect warm-up strategy)

Symptoms

  • Check Engine Light (MIL) illuminated and P0128 stored
  • Engine takes a long time to reach normal operating temperature
  • Cabin heater produces weak or cold airflow
  • Reduced fuel economy and possibly richer-than-normal running during warm-up
  • Temperature gauge reads lower than normal or fluctuates
  • Cooling fans running more often than expected

What to check

  • Scan tool: read live coolant temperature (ECT) and IAT, freeze-frame data and readiness status
  • Inspect coolant level and reservoir for proper fill and contamination
  • Visually inspect wiring and connectors at ECT and IAT sensors for corrosion or damage
  • Confirm cooling fan operation and verify fan is not running continuously at low temp
  • Check for recent coolant service or air in system (signs of bubbles, recent repairs)
  • Compare heater output at dash (heater core warm-up) to coolant temperature readings

Signal parameters

  • Typical normal coolant operating temperature: 80–105 °C (176–221 °F) depending on model and thermostat
  • Thermostat opening (typical) ~85–95 °C (185–203 °F) — model dependent
  • Fans typically command ON around 95–105 °C (203–221 °F) (varies by model/engine)
  • Expected warm-up: coolant should rise steadily from ambient and reach ~70–90 °C within several minutes at idle or short drive — if it stays near ambient, suspect stuck-open thermostat or sensor fault
  • ECT sensor resistance/voltage: compare to manufacturer table with scan tool or multimeter (expected voltage decreases as temp rises; typical 0.5–4.5 V/OTC varies)

Diagnostic algorithm

  1. Connect a calibrated scan tool, read freeze-frame and live data: record coolant temp at cold start and after 5–10 minutes of idle/driving.
  2. Verify ambient temperature and compare expected warm-up time to observed data.
  3. Inspect coolant level, top up if low, and bleed air from system if required. Re-test warm-up behavior.
  4. Check ECT sensor: inspect connector and wiring for corrosion, measure sensor resistance/voltage and compare to spec. Wiggle test wiring while monitoring live data for intermittent faults.
  5. Confirm IAT sensor readings are plausible; replace or repair if out of range and affecting warm-up strategy.
  6. Verify thermostat operation: with engine cold, start and monitor temperature increase and feel upper and lower radiator hoses (use caution). A stuck-open thermostat shows little temperature rise/low delta between hose and engine block.
  7. If thermostat suspect, perform a controlled check by removing thermostat (if accessible) or replacing with correct OEM thermostat and retest.
  8. Verify cooling fan control: ensure fans are not commanded ON at low temps due to failed relay/module; check fan relay, temperature switch or control module.
  9. Inspect for restricted coolant flow: check water pump operation, heater core/hoses and radiator for blockages.
  10. After repairs, clear codes, perform a warm-up/drive cycle and confirm code does not return and readiness monitors set.

Likely causes

  • Thermostat stuck open (most common on Audi with long warm-up)
  • Faulty ECT sensor or intermittent connector/circuit short to ground
  • Low coolant level or air pocket after recent service
  • Cooling fan relay/stuck fan causing overcooling
  • Dirty or degraded coolant reducing heat transfer efficiency
  • IAT sensor fault feeding incorrect intake temperature to the ECU (less common)

Fault status

⚠️ Status
P0128 — Engine coolant temperature below thermostat regulating temperature; warm-up not achieved within expected time.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0125 P0117 P0118 P0112 P0113 P0480
406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

MITSUBISHI

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