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.
  CAUTION: After servicing or replacing faulty parts, perform Clear Memory Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Clear Memory Mode>OPERATION . and Inspection Mode Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Inspection Mode>PROCEDURE .

1. CHECK FOR ANY OTHER DTC ON DISPLAY. 

   Is any other DTC displayed?

   • Yes  : Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>List of Diagnostic Trouble Code (DTC) .
   • No  : Go to 2. 
2. CHECK ENGINE COOLANT. 

   Is the engine coolant amount normal?

   • Yes  : Go to 3. 
   • No  : Refill the engine coolant. Ref. to COOLING(H4DOTC)>Engine Coolant .
3. CHECK RADIATOR FAN. 
   1. Start the engine.
   2. Check the operation of the radiator fan while keeping idling (acceleration OFF) for approx. 3 minutes.

   Does the radiator fan continuously rotate for 3 minutes or more during idling?

   • Yes  : Repair radiator fan circuit. Ref. to COOLING(H4DOTC)>Radiator Main Fan and Fan Motor . Ref. to COOLING(H4DOTC)>Radiator Sub Fan and Fan Motor .
   • No  : Go to 4. 
4. CHECK ENGINE COOLANT TEMPERATURE SENSOR. 

   Check the engine coolant temperature sensor.

   Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>Engine Coolant Temperature Sensor>INSPECTION .

   Is the check result OK?

   • Yes  : Replace the thermostat. Ref. to COOLING(H4DOTC)>Thermostat .
   • No  : Replace the engine coolant temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>Engine Coolant Temperature Sensor .