DTC P0128: Coolant Thermostat (Engine Coolant Temperature Below Thermostat Regulating Temperature): Notes

WARNING: This page is about a different variant/trim than selected.

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DTC detecting condition:

Detected when two consecutive driving cycles with fault occur.

Trouble symptom:
- Thermostat remains open.
- Coolant temperature rises slowly.
NOTE: After the faulty parts are repaired or replaced, perform the final check in Basic Diagnostic Procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.0 (EXCEPT FOR HEV))>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

CHECK DTC .
Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAG)>DIAGNOSTIC TROUBLE CODE (DTC) .

Is DTC other than P0128 displayed? (Current malfunction)

Yes: Check the appropriate DTC using the "Diagnostic Trouble Code (DTC)". Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.0 (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .

No: Go to 2.
CHECK ENGINE COOLANT .
Check the engine coolant level.

Is the check result OK?

Yes: Go to 3.

No: Refill the engine coolant. Ref. to COOLING (H4DO (EXCEPT FOR HEV))>ENGINE COOLANT .
CHECK RADIATOR FAN .
1. Start the engine.
2. While keeping idling for approx. 3 minutes (with accelerator OFF), check the operation of the radiator fan.
  Does the radiator fan continuously rotate for 3 minutes or more during idling?
  
  Yes: Repair radiator fan circuit. Ref. to COOLING (H4DO (EXCEPT FOR HEV))>RADIATOR FAN AND FAN MOTOR ASSEMBLY .
  
  No: Go to 4.
CHECK ENGINE COOLANT TEMPERATURE SENSOR .
Check the engine coolant temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO (EXCEPT FOR HEV))>ENGINE COOLANT TEMPERATURE SENSOR>INSPECTION .

Is the check result OK?

Yes: Replace the thermostat. Ref. to COOLING (H4DO (EXCEPT FOR HEV))> THERMOSTAT .

No: Replace the engine coolant temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO (EXCEPT FOR HEV))>ENGINE COOLANT TEMPERATURE SENSOR .