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

1. - 

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

Trouble symptom: 

• Thermostat remains open
• Coolant temperature rises slowly

1. CHECK DTC  .
   1. Turn the ignition switch to ON.
   2. 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)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .

      No  : Go to  2 .

2. CHECK ENGINE COOLANT  .

   Check the engine coolant. Ref. to COOLING (H4DO)>ENGINE COOLANT .

   Is the check result OK?

   Yes  : Go to  3 .

   No  : Refill the engine coolant. Ref. to COOLING (H4DO)>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 the radiator fan system. Ref. to COOLING (H4DO)>RADIATOR FAN SYSTEM .

      No  : Go to  4 .

4. CHECK ENGINE COOLANT TEMPERATURE SENSOR (INLET AND OUTLET)  .

   Check the engine coolant temperature sensor (inlet and outlet). Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>ENGINE COOLANT TEMPERATURE SENSOR .

   Is the check result OK?

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

   No  : Replace the engine coolant temperature sensor (inlet and outlet). Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>ENGINE COOLANT TEMPERATURE SENSOR .