Error Code P0128 is described as a Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).


Also referred as “Thermostat Below Regulating Threshold”, Error Code P0128 means the PCM (Powertrain Control Module) of the vehicle has detected a problem on the thermostat, as it failed to reach the required temperature level within the specified time after starting the engine.

The purpose of the code is to point out a faulty thermostat. And to determine whether the engine has reached a ‘normal’ temperature or not, it calculates and takes into account the duration of time the vehicle has been running, the IAT (Intake Air Temperature) sensor reading, the ECT (Engine Coolant Temperature) sensor reading, and the speed of the vehicle.

The role of the Error Code P0128 is to track the amount of time it takes for the engine to reach and maintain its right operating temperature. This enables the fuel system to run in a ”closed loop” condition, which means the two or more Oxygen Sensors found in the Exhaust system influence the Air Fuel Ratio.

Common Symptoms

  • Check Engine light is on
  • Vehicle may be hard to shift, or not at all at highest gear during freeway speed
  • Excessive fuel consumption

Possible Causes

  • Faulty Engine Thermostat
  • Faulty Engine Coolant Temperature Sensor
  • Faulty Intake Air Temperature Sensor
  • Faulty Cooling System
  • Low Engine Coolant
  • Dirty Engine Coolant which results to wrong Coolant Temperature Sensor readings
  • Faulty Engine Cooling fans (always running)

How to Check

When Error Code P0128 appears in the PCM, the engine is doing either of the two things: the engine takes too much time to warm up  to its require operating temperature (takes more than 15 minutes) as measured by the Coolant Temperature sensor, or the operating temperature is below 160° – 170° F range throughout the operation of the vehicle. This will cause the Fuel System to go back to warm up mode, which is a richer mixture. As a result, this causes higher Co2 and HC levels being emitted out of the vehicle’s pipeline, and increase in fuel consumption.

To diagnose:

First step

It is important to take note and record the vehicle’s freeze frame data to find out which of the operational modes triggered the error code. This means you need to pay attention to the vehicle’s engine load, MPG, RPM, and TPS, and of course, both the Engine Coolant Temperature and Intake Air Temperature. These values will help determine whether the vehicle was being driven on a freeway or slower speeds. The distinction between the two will help verify the code.

Second Step

Plug in your OBD II scanner tool to the vehicle and choose the most factory-like data stream for engine sensors. Start the engine and take note of how the Coolant Temperature values change with the heater turned off.

Third Step

If the readings you get in the Coolant Temperature will not go beyond 160° – 170° F in the span of 15 minutes, then try to put your foot on the brake, shift to Drive and then press the throttle for around two to three minutes and see if the readings increase. If the Coolant Temperature readings reach the right range, then the fault is most likely in your Thermostat.

Fourth Step

If the vehicle still can’t reach the 160° – 170° F range, turn off the heater and increase the RPM to around 2000 while in Park. If the readings from the Coolant Temp fall out of required range, then the Thermostat is the culprit.

When diagnosing for this code, make sure you verify both the electric and mechanical cooling fans are not stuck at “On” position, as this will cause the engine to run at very low operating temperature. Also, make sure you verify the readings of your Intake Air Temperature sensor are right, not too hot or too cold in relation to both air temperature and under-hood air temperature. The good rule of thumb here is to get the Air Temperature at 100° F below the Coolant Temperature readings after warmup. The reading values should be nearly identical when cold starting the vehicle.

How to Fix

The obvious and easy solution is to replace the thermostat. However, there are other ways to fix this code as well.

  • Check the strength and level of the coolant
  • Check the right operation of the cooling fan, check whether it is running than it should. Replace if needed.
  • Check the ECT (Engine Coolant Temperature) sensor operation, replace if needed.
  • Check the IAT (Intake Air Temperature) sensor operation, replace if needed.
  • If all the above items are in good condition, then you may need to replace the thermostat.
  • For Nissan vehicles, check for the TSB (Technical Service Bulletins), as the ECM may have to be reprogrammed to fix the error code.

Parting Tips

Other related error codes are P0115, P0116, P0117, P0118, P0119, and P0125. For more information about these error codes, check out our website.