WOMS
P code cooling Low severity

P0128 Coolant Thermostat (Coolant Temp Below Regulating Temp)

The engine coolant is not reaching the expected operating temperature within the expected time.

Definition

P0128 sets when the PCM determines, based on coolant temperature sensor data and engine run time, that the engine took too long to warm up or never reached its expected operating temperature. The most common cause is a thermostat stuck open.

Common causes

  • Thermostat stuck open
  • Failed coolant temperature sensor
  • Low coolant level
  • Excessive cooling in extreme cold weather

Common misdiagnoses

  • Replacing the coolant temp sensor when the thermostat is the actual cause.

Troubleshooting steps

  1. 1. Compare actual coolant temp to spec

    Coolant should reach ~180-205°F within ~10 minutes of running.

  2. 2. Test thermostat

    Remove and bench-test in hot water — it should close at room temp and open at the rated temperature.

  3. 3. Verify coolant temp sensor

    Compare sensor reading to an IR thermometer reading on the thermostat housing.