Q&A

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 PCM detected the ECT sensor did not reach 167ºF after a normal warmup period of 5-10 minutes expired. The PCM estimates the coolant temperature based on the engine temperature at startup, the engine loads and the engine speed. The PCM then compares the estimated temperature with the actual ECT signal. When the estimated coolant temperature reaches 167º F, the PCM checks the actual ECT signal. If the actual ECT signal is less than 167º F, the PCM interprets this as a fault in the thermostat or the engine cooling system and sets DTC P0128.

This code sets in 2 Trip(s).
This code will turn on the MIL (Malfunction Indicator Lamp)

REPLACE THE THERMOSTAT.