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P0128 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


Having a P0128 trouble code is a sign that your coolant thermostat is registering an issue. The problem is that the coolant temperature is below the thermostat regulating temperature. 


In essence, the P0128 trouble code means that the engine coolant is not getting warm enough, quickly enough. What this boils down to is that there is a problem with the engine not reaching its operating temperature within a certain amount of time. The engine control module (ECM) is what determines and governs the length of time that this takes, based on the ambient temperature inside of the vehicle


There are several common issues that can cause a P0128 trouble code: 

The absolutely most common cause is that the engine coolant thermostat has gotten stuck open or is operating prematurely. The second most probable cause is that the genuine coolant temperature sensor or the wiring connected with the sensor is faulty or damaged. Both or either of these issues could cause a P0128 trouble code to occur. 

Also Read: Why Is My Temperature Gauge Not Going Up?


There are two common symptoms that will occur with a vehicle is you are experiencing a P0128 trouble code: 

  • The heater is not being hot enough, due to the coolant temperature in the engine being low. 
  • The car takes too long to warm up or you notice that the temperature gauge does not go up as high as it used to.

If you notice either of the symptoms when testing or test-driving a vehicle, this may be an indication or validation of a P0128 trouble code. 


There are a series of logical steps that a mechanic will take to properly diagnose and confirm a P0128 trouble code. Keep in mind that diagnostic methods and measures can vary from mechanic to mechanic, but overall there will be a certain set of steps that each mechanic should be following to assess this specific trouble code: 

  • The first thing you should do is check to see how hot the coolant temperature is when the coolant starts flowing through the radiator hose, to the thermostat. 
  • In general, the hose should be barely warm until the thermostat opens. 
  • When the thermostat opens, coolant that is very hot should start to flow and rapidly warm up the radiator hose. You need to be cautious because it is hot enough that you may get burned.
  • If you notice that the radiator hose slowly warms up and it doesn’t quickly or suddenly become hot, that is indicative that the thermostat is stuck open. If the hose gets hot quickly, like it should, the next thing you need to check is the reading you get from the coolant temperature sensor.  
  • To make sure your reading is accurate, you should use the scan tool, but you can also use your temperature gauge on your car. Compare the reading that you get to the one that you obtain by an infrared thermometer. If everything is working properly, the thermometer should read around 200 degrees Fahrenheit, normally. If the scan tool reading does not match the thermometer reading (or is at least close to it) then there is an issue with the sensor inside of the car. 
  • If the sensor is the problem, the connectors will typically deteriorate. A poor connection can be the source of this problem, and lead to further issues with the vehicle. If the connector appears to be okay and working, then the sensor could be the problem. 
  • In rare cases of a P0128 trouble code, there could be a problem with the supply voltage supplied by the ECM. It is difficult to get into detail on this because you are going to need factory-level information that is specific to your vehicle, and varies from car to car. You need a knowledge of electronic diagnostics to prevent causing damage to the vehicle.

Common mistakes

The most common mistake you can make when addressing a P0128 trouble code is replacing the thermostat without properly checking the rest of the vehicle

How serious is this?

This is relatively not a serious trouble code, but keep in mind that if the vehicle is used in this condition over a long period of time, it could cause greater issues. There is a small chance that water contamination of the oil could cause deterioration and engine damage. 

With this code being triggered by the engine coolant not being hot enough, it means the engine is not getting hot enough for the engine condensation to be burned off. If that’s the case, water can end up in the oil. Even though this particular issue of condensation may not result in noticeable damage until years later, the chance of damaging your engine is still there. 

What repairs can fix the code?

repair manuals

The easiest repairs that you need to fix the code are to: 

  • Replace the thermostat. This is the most common repair by far. 
  • Replacing the coolant temperature sensor, the wiring, and/or the connector that is associated with the sensor being damaged.

Related codes

None listed.


If you believe you may have a P0128 trouble code, it is something that should be assessed and confirmed as soon as possible. Even though there is a low likelihood that this will cause severe damage to your vehicle, the chance is still there.