P0128 Thermostat OBD-II Trouble Code

Technical Description

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Brands of vehicles that may see this code include but are not limited to Pontiac, Toyota, Mazda, Dodge, Chevrolet, Honda, Jeep, Ford, VW, etc. Although generic, the specific repair steps may vary depending on make/model.

This means that the engine’s powertrain control module (PCM) has detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. Often, the intent of the P0128 code is to indicate a faulty thermostat (likely stuck in the open condition). Similar codes: P0125

In determining that the engine did not reach a “normal” temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.


You will likely not notice any drivability problems other than the MIL illumination. In some cases you may experience symptoms such as:

  • Engine takes longer to warm up
  • Engine temperature drops while driving at higher speeds

A photo of a typical thermostat: P0128 Thermostat


A code P0128 may mean that one or more of the following has happened:

  • Low engine coolant level
  • Leaking or stuck open thermostat (likely)
  • Faulty cooling fan (running too much)
  • Faulty coolant temperature (ECT) sensor
  • Faulty intake air temperature (IAT) sensor

Possible Solutions

Note: Please be sure the engine is turned off and cool when doing work on the cooling system. When the engine is warm, the cooling system is pressurized and very hot and can cause burns or other personal injuries if you are not careful.

Past experience indicates that the most likely solution for a P0128 is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

  • Verify coolant strength & level
  • Verify proper cooling fan operation (check if it’s running more than it should). Replace if necessary.
  • Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
  • Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
  • If the above items check out good, replace the thermostat
  • If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Regardless of what vehicle year, make and model, it is advisable to check for TSBs because if there is a known fix set out by the automaker, it can save you time and money in the diagnosis and repair.

An example of the importance of checking for TSBs or technical service bulletins, is if you have a Honda Accord, bulletin #01-164 may apply to you. TSBs are also called manufacturer communications.

Other ECT engine coolant sensor and circuit related DTCs include but are not limited to: P0115, P0116, P0117, P0118, P0119, P011A, P011B, P0125, and P0126.

p0128 Diagnosis & Repair Video

We found this helpful video regarding diagnosis of the p0128 code. We are not affiliated with the producer of this video, it is here as a convenience to our visitors:

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