P1408 — Coolant heating circuit fault | Heater circuit fault

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.

Similar codes