DTC P0128: Thermostat Rationality: Testing

WARNING: Never open the cooling system when the engine is hot. The system is under pressure. Extreme burns or scalding may result. Allow the engine to cool before opening the cooling system.

NOTE: If any Engine Coolant Temperature (ECT) sensor, Ambient Air Temperature (AAT) sensor, Camshaft Position (CMP) sensor or Crankshaft Position (CKP) sensor DTCs have set along with P0125, diagnose them first before continuing. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . Ensure that the ECT, AAT, CMP or CKP sensors are operating properly before continuing.

Check the coolant system to make sure that the coolant is at the proper level. If the coolant level is low, check for visible leaks or preform a pressure test on the coolant system. Is the coolant level low or were any leaks found? If yes, repair coolant leak and add the necessary amount of coolant. If no, replace the thermostat and add the necessary amount of coolant.