Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)


The P0128 indicates that the PCM has received a coolant temperature reading from the ECT sensor that is lower than normal, after a significant period of engine operation.

Code Set Parameters

ECT sensor readings that occur after the OBD-II system has gone into closed loop operation, and are lower than the manufacturer’s specified value, will cause a trouble code to be stored and a malfunction indicator lamp to be illuminated.


In most cases there will be no noticeable symptoms and possibly not even a service engine soon lamp (until the third drive cycle - which may or may not occur).

Common Causes

This could be caused by insufficient engine warm-up time, a faulty coolant temperature sensor, a low engine coolant condition, or a faulty thermostat.

Common Misdiagnosis

The most common misdiagnosis is replacement of the ECT sensor, when a low engine coolant level is to blame.


  • First, check to make sure that the engine is full of a suitable coolant ant that the thermostat is operating properly
  • Begin your diagnosis by visually inspecting all wiring and connectors
  • Look for shorted or burned wiring and replace circuitry and connectors as required
  • If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Check the fan clutch (if equipped)
  • This code is frequently set because the driver does not allow sufficient engine warm up time (especially in cold weather)
  • Resetting the code and seeing if it returns is a good idea when this code is displayed.