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

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

Trouble symptom: 

• Thermostat remains open.
• Coolant temperature rises slowly.

1. CHECK DTC. 

   Read the engine DTC using the Subaru Select Monitor or general scan tool. Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC other than P0128 displayed? (Current malfunction)

   Yes:  Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>LIST OF DIAGNOSTIC TROUBLE CODE (DTC) .

   No:  Go to  2.

2. CHECK ENGINE COOLANT. 

   Check the engine coolant.

   Is the check result OK?

   Yes:  Go to  3.

   No:  Refill the engine coolant. Refer to COOLING (EXCEPT FOR STI)>ENGINE COOLANT .

3. 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. Refer to COOLING (EXCEPT FOR STI)>RADIATOR MAIN FAN AND FAN MOTOR . Refer to COOLING (EXCEPT FOR STI)>RADIATOR SUB FAN AND FAN MOTOR .

   No:  Go to  4.

4. CHECK ENGINE COOLANT TEMPERATURE SENSOR. 

   Check the engine coolant temperature sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>ENGINE COOLANT TEMPERATURE SENSOR>INSPECTION .

   Is the check result OK?

   Yes:  Replace the thermostat. Refer to COOLING (EXCEPT FOR STI)>THERMOSTAT .

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