How to Fix Code P0128: Solution for Faulty Engine Coolant Temperature Sensor

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When the check engine light illuminates your dashboard, one of the first things you should do is scan the code using an OBD-II scanner. If P0128 is the displayed code, you should address it as soon as possible.

This article will discuss the causes, symptoms, and how to fix the P0128 trouble code possibly. Read on!

What is Code P0128?

The error code P0128 is triggered when the coolant thermostat temperature is below the regulating temperature. When the Engine Control Module (ECM) or the Powertrain Control Module (PCM) detects a cooler temperature lower than its normal operating range, the code P0128 is set, and the Check Engine Light comes on.

Symptoms of Code P0128

One of the first indications you will notice is an illuminated check engine light. Here are other symptoms you might have sooner or later:

  • Lack of power, poor fuel economy, or engine overheating.
  • Your car will take too long to warm up than usual.
  • The temperature gauge will not go up as it used to.

Causes of Code P0128

To fix this error code, thoroughly diagnosing all the possible causes is required. There are a few things that can cause this code to be set. Here are some of the common causes:

  • Malfunctioning coolant temperature sensor
  • A clogged radiator cooling fan
  • Low engine coolant level
  • Incorrect thermostat setting
  • The engine coolant thermostat has stuck open or is opening prematurely.
  • Faulty wiring is associated with the sensor.

How to Diagnose Error Code P0128? 

Diagnosing the P0128 code is crucial to determining the root cause of the problem. Remember that diagnosing is not always easy to perform, so call for a professional mechanic if you aren't sure of what to do. These are the things you can check, such as:

Check the Coolant Temperature 

Check the hose connected to the thermostat. If the radiator hose is getting hot, you should check the reading of the coolant temperature sensor. You can use a thermometer or scan tool. Compare the infrared thermometer reading with the scan tool's reading. 

Typically, the temperature should be around 200 Fahrenheit. If the temperature does not match with the scan tool reading results, then there is an issue with the thermostat sensor of the car.

Inspect for Electrical Problems 

Visually check the electrical wiring harness and connectors. Sometimes, the connector might not be plugged in all the way, or it could be corroded. You can also inspect for melted wires or signs of arcing.

Test your Engine's Cooling Fan

You can test your engine's cooling fan by turning on your car and letting it idle for about ten minutes. If the fan doesn't come on, it might be a problem. You can either replace the fan or clean the blades to address it.

You should also check the fan if it is stuck open because this may cause to cool the engine excessively and store the P0128 code. If you have already checked all of these things and still get the P0128 code, it might be time to replace the coolant thermostat.

Replace the Coolant Thermostat

After diagnosing and repairing all the possible causes of the P0128 code, and nothing clears the error code, you may need to replace the coolant thermostat. It's a relatively easy job you can do at home with some basic hand tools. 

However, if you think you can do it yourself, please be advised that every car model may vary slightly, so always consult your car's service manual for specific instructions on how to replace the coolant thermostat in your vehicle.

Warning: If you decide to replace the coolant thermostat on your own, make sure the engine is cool and that you have drained all of the coolants from the system. Failure to do this can cause injury.

How to Fix Code P0128?

If you did the diagnosing yourself, you might probably be tempted to repair independently. Yes, you can do it yourself; however, please be advised that mistakes can be costly.

So, if you are not confident in your skills or don't have the time, take it to a professional. But since we want to help you out, here are some possible fixes to clear an OBDII trouble code P0128:

Step-by-Step Guide to Replacing the Thermostat

  • First, you need to remove the thermostat. To do this:
  • Drain the coolant
  • Disconnect the radiator hose by loosening it from its retaining clip.
  • Remove the two bolts that are mounted to the engine.
  • Replace the thermostat gasket if needed.
  • Torque the bolts before securing the coolant hose back to the retaining clip.
  • Reset the check engine light to see if you have made a successful repair.

Step-by-Step Guide to Replacing the Coolant Temperature Sensor

  • If you have an aftermarket coolant temperature sensor, you can replace it without removing the                       thermostat.
  • Disconnect the electrical cable and unscrew the sensor from its mounting bracket.
  • Install the new sensor by reversing these steps and resetting your check engine light.

Step-by-Step Guide to Flushing your Cooling System

A more drastic solution that can often take care of the issue is to flush your entire cooling system. This will remove any built-up sediment or corrosion and might be necessary if you've already replaced the thermostat and coolant temperature sensor. To do this:

  • If your car has a mechanical cooling fan, you'll need to remove it. There are usually four bolts holding             fan in place.
  • Disconnect the hoses and wires attached to the radiator and loosen the clamps securing them.
  • You can now pull the entire radiator straight out.
  • Once it's out, take it to a radiator shop and have them flush it for you.
  • Once the fan is clean, put it all back together and start your car. Hopefully, the P0128 code will be gone!

Common Mistakes When Diagnosing the P0128 Code

The P0128 code is associated with many symptoms, so diagnosing it can be challenging. As a result, we can't avoid making some common mistakes. Here are some mistakes people make when trying to fix this code are:

  • Not properly cleaning the radiator fan
  • Not flushing the cooling system
  • Replacing the wrong part
  • Not checking all the possible cause
  • Replacing the coolant temperature sensor without confirming its cause.
  • Replacing the thermostat without inspecting the vehicle

P0128 Code FAQs

1. Is the P0128 Code Serious?

P0128 is not as severe as we think; however, it can become one if not properly fixed. When the coolant is not getting enough heat for the condensation that the engine needs to burn off, that water could end up in the oil, which can damage the engine. So if you can address it as soon as possible, you can save your car and a lot of money.

2. Is it okay to drive with the P0128 code?

Yes, you can continue to drive your vehicle with the P0128 code. However, it is crucial to have the issue fixed as soon as possible to avoid accidents from happening.

3. How to know if the problem is with the sensor or something else?

If you have replaced the sensor and the code persists, then you may have a problem with the wiring or the computer. You need to take your vehicle to a mechanic for further diagnosis.

4. Do I need to use a specific device to clear the P0128 code?

You can use a generic scan tool or a code reader to clear the P0128 code.

5. How much does the diagnosis and repair cost?

The cost for diagnosis and repair will vary depending on the mechanic you go to. However, it is typically a relatively inexpensive fix. Some mechanics usually charge around $220-$250 for the thermostat and $140-$190 for the coolant temperature sensor.


The code P0128 is not as serious as we think; however, you should have it fixed as soon as possible to avoid an engine failure. So, if you think your car might have this issue, address it immediately! Did this blog post help you understand the symptoms of code P0128 and how to fix them? Let us know in the comments below!