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

Courtesy of SUBARU OF AMERICA, INC.

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 (HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .

No  : Go to  2.

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

Is the check result OK?

Yes  : Go to  3.

No  : Refill the engine coolant. Ref. to COOLING (H4DO (HEV))>ENGINE COOLANT .

1. Turn the ignition switch to ON.
2. Navigate to Inspection Mode for measuring Exhaust Gas. Ref. to P-HEV HYBRID POWERTRAIN CONTROL (DIAGNOSTICS)>WORK SUPPORT>OPERATION>SERVICE MODE .
   NOTE: If it is necessary to run the engine for 10 minutes or more, fasten the driver's seat belt.
3. Start the engine.
4. 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 (HEV))>RADIATOR FAN SYSTEM .

   No  : Go to  4.

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

Is the check result OK?

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

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