Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

You will not usually see any drivability issues with this other than the check engine light (CEL).

You may find one or more of the following issues:

  • Low engine coolant level
  • Leaking or stuck open thermostat
  • Continuously running coolant fan
  • Faulty coolant temperature sensor
  • Faulty air inlet temperature (IAT) sensor

Possible fixes:

  • Verify coolant level and strength
  • Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
  • Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
  • Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
  • If the above items check out good, replace the thermostat
  • If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Other ECT sensor and circuit related DTCs: P0115, P0116, P0117, P0118, P0119, P0125

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