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P0128 Fault Code: What It Means and How to Fix It

If you’re a DIY car enthusiast, you might have encountered the dreaded P0128 code. This is a common code that pops up on OBD-II scanners, indicating that there’s a problem with the coolant thermostat. But what exactly is the P0128 code? What does it mean? And how can you fix it?

I’m John Cunningham, a qualified mechanic, you are in the right place, and very shortly, you’ll have a good understanding of the cause, the diagnosis process, and the fix.

In this short explainer, we’ll cover the following:

What is the P0128 code?

The P0128 code is a generic OBD-II code that indicates that the engine coolant temperature is below the thermostat regulating temperature. In other words, the engine is not reaching its normal operating temperature, which can affect its performance and fuel efficiency.

What does the thermostat do?

Coolant system info graphic

The thermostat (stat) is a small valve located between the engine and the radiator that regulates the flow of coolant through the engine.

It’s designed to open and close at specific temperatures to maintain the engine’s optimal operating temperature. When the engine is cold, the thermostat remains closed, preventing coolant from flowing through the engine. This allows the engine to warm up quickly and reach its optimal operating temperature.

Once the engine reaches its optimal temperature, the thermostat opens, allowing coolant to flow through the engine and regulate its temperature.

P0128 Symptoms

Here are the top 7 symptoms associated with this fault code:

  1. Engine light is on: When the P0128 code appears, the check engine light will illuminate the dashboard. This is the most obvious symptom and is the first indication that something is wrong.
  2. Temperature gauge reads low: Another symptom of this fault code is a low reading on the temperature gauge. If the gauge is showing that the engine is not warming up to its normal operating temperature, it could be a sign of a problem with the thermostat or the cooling system.
  3. Poor fuel economy: If the engine is not reaching its optimal operating temperature, it may not be running efficiently. This can result in decreased fuel economy.
  4. Engine runs poorly: If the engine is not at the right temperature, it may not be running as smoothly as it should. You may notice that the engine is running rough or stalling.
  5. Heater doesn’t work: If the engine is not warming up to its normal operating temperature, it may affect the heater’s ability to produce warm air. If the heater is blowing cold air, it could be an indication of a problem with the engine’s thermostat.
  6. Emissions test failure: When the engine is not running at its optimal temperature, it can cause the vehicle to fail emissions tests. This is because the engine may not be burning fuel as cleanly as it should, resulting in increased emissions.
  7. Reduced engine power: If the engine is not running at its optimal temperature, it may not be producing as much power as it should. You may notice a decrease in acceleration or overall power from the engine.

What causes the P0128 code?

Engine coolant system

The P0128 code can be caused by a variety of factors, including the following:

  1. A faulty thermostat: If the thermostat is stuck open, coolant will flow through the engine all the time, which can cause the engine to run cooler than normal.
  2. Low coolant levels: If there isn’t enough coolant in the system, the engine won’t be able to reach its optimal operating temperature.
  3. A faulty coolant temperature sensor: The coolant temperature sensor sends information about the engine’s temperature to the engine control module. If the sensor is defective, it may send incorrect information, which can cause the P0128 code to appear.

How do you diagnose the P0128 code?

To diagnose the P0128 code, you’ll need an OBD-II scanner to read and clear codes. Here’s what you need to do:

  1. Use the OBD-II scanner to read the code and any other codes that may be present.
  2. Check the coolant level and make sure it’s at the appropriate level.
  3. Check the thermostat to see if it’s functioning properly. You can do this by removing the thermostat from the engine and placing it in a pot of boiling water. If it opens, it’s working correctly. If it doesn’t, you’ll need to replace it.
  4. Check the coolant temperature sensor to see if it’s functioning correctly. You can use a multimeter to test the sensor’s resistance. Refer to your car’s service manual for the appropriate values.

I wrote a post all about testing and replacing the ECT sensor, and you can check it out here – “Can you drive with a bad ECT?”

How do you fix the P0128 code?

Car Thermostat

If you’ve diagnosed the problem and found the thermostat or coolant temperature sensor is faulty, you must replace them.

Here’s how to do it:

  1. Drain the coolant from the system.
  2. Remove the thermostat housing and thermostat.
  3. Install the new thermostat and thermostat housing.
  4. Refill the system with coolant.

I wrote a whole post about testing and replacing a temperature sensor, and you can check it out here “Coolant sensor testing.”

If you’d like more information on thermostats, I wrote a whole post about them, and you can check that out here “What’s a thermostat.”

About the Author

This article was created with the assistance of AI technology to aid the author, John Cunningham, who is a seasoned Red Seal-certified auto technician with more than 25 years of experience in vehicle repairs. However, please note that John Cunningham has edited the content to ensure accuracy and quality.

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