Q&A

 Master

replace the thermostat.

Roy

 Master

P0128 Thermostat System MalfunctionPossible Causes Setting Conditions
•Check for low coolant level for an incorrect coolant mixture
•Check the operation of the thermostat (it may be stuck open)
•ECT sensor signal circuit has high resistance (it may be "skewed")
•ECT sensor has failed
Engine running; ECT sensor signal less than 140ºF at startup, and the ECM detected the ECT sensor did not reach 167ºF after a normal warmup period of 5-10 minutes expired. The ECM estimates the coolant temperature based on the engine temperature at startup, the engine loads and the engine speed. The ECM then compares the estimated temperature with the actual ECT signal. When the estimated coolant temperature reaches 167º F, the ECM checks the actual ECT signal. If the actual ECT signal is less than 167º F, the ECM interprets this as a fault in the thermostat or the engine cooling system and sets DTC P0128.