What Does The P0128 Code Mean On Duramax? How to Fix?

The P0128 code is a common diagnostic trouble code (DTC) on Duramax engines. It’s caused by low engine coolant temperature, which can lead to poor engine performance and reduced fuel efficiency.

In this article, we’ll explain what the P0128 code means, how to diagnose it, and how to fix it.

What is the P0128 Code?

The P0128 code stands for “Engine Coolant Temperature Below Thermostat Regulating Temperature.” This code indicates that the coolant in your engine is not reaching its normal operating temperature.

The thermostat regulates the engine’s temperature by opening and closing as needed based on signals from your vehicle’s sensors.

When these sensors detect that the coolant is too low, they will trigger the check engine light with a P0128 code.

Here is a break-down of this code for your convenience:

P: Powertrain. The problem is with the engine or the transmission.
0 – The problem is general and not caused by the manufacturer’s .
1 – Air/fuel mixture
28 – Fault Index ( Coolant temperature lower than thermostat temperature)

What Causes The P0128 Code?

In most cases, this code is caused by a faulty coolant temperature sensor or thermostat.

If either of these components fail, they can cause your engine to run too cold and trigger the check engine light with a P0128 code.

Other possible causes include air leaks around the intake manifold gasket or an old radiator cap that needs replacing. Read in details below:

Some of the most common causes of P0128 are:

Fan that doesn’t work

The radiator fan’s main job is to cool down the car’s engine and coolant. But if it doesn’t work right or works in a strange way, there could be problems.

When the fan doesn’t work as it should, the coolant usually starts to heat up slowly.

Inadequate Coolant

The level and quality of the coolant can also cause the P0128 code. Under no circumstances should the level of coolant be lower.

When there aren’t enough coolants, it’s one of the main reasons why the thermostat won’t move. Also, the dirty coolant has a big part to play in this.

Faulty Sensor

If the sensors that take accurate temperature readings are broken, you’ll see the P0128 code more often.

Even if there is nothing wrong with the engine coolant, this code can show up because the wiring isn’t good and the sensor is prone to making mistakes.

Malfunctioning PCM

Not many things go wrong with the PCM. It doesn’t happen very often, but when it does, you’ll notice that the code shows up a lot.

Most of the time, these things happen faster because software updates have bugs or need to be updated right away.

If you fix the problems listed above, the error code p0128 will go away. But you would need help from a mechanic to fix most of these problems.

It will cost you more money on top of the cost of the parts that need to be changed.

How Do You Diagnose And Repair A P0128 Code?

The P0128 code can be fixed at home without the help of a pro. You can get rid of the extra cost for the mechanic if you want to.

And here are the steps you need to take for this level of coolant:

Step 1: Make sure the error is real

First, you should always check to see if there is an error. Until you have solid proof, guesses about the error don’t mean much.

Because of this, you need an OBDII scanner. All you have to do is hook up the scanner to the system in your car, and then you can scan.

When it’s done, you’ll see the error code numbers that show what’s wrong with the Duramax engine in your vehicle.

Step – 2: Examine ECT Sensor

Most of the time, ECT sensors are hooked up to the thermostat. Also, if they are broken or broken down, they need to be changed.

First, you need to find it so you can see if it’s been damaged. You can use a multimeter to check if the output is good enough.

If the resistance is not between 2000 and 3000, you need to replace it.

Step 3: Inspect Thermostat

You should check to see if the thermostat is working right. But before you check, you should be very careful because the engine is too hot.

If the thermostat doesn’t work right, you need to get a new one. Place the new one correctly and carefully connect its end to the engine when setting it up.

Step – 4: Clear the Error Code

After you’ve double-checked and fixed every problem, you’ll need to start clearing the code by hand.

Again, you have to use the OBDII scanner to clear the codes and start the command by telling the scanner to clear out the codes that aren’t being used.

After the clearing is done, run another scan to see if the problem is still there or not. If the problem keeps coming back, you should call a mechanic.

Other Signs That Your Duramax Seems to have P0128

You already know what caused the P0128 code and what it means. But a P0128 code in the Duramax engine could be caused by some other signs. Here are a few of the most well-known:

  • Uses a lot of fuel
  • Temperatures are falling quickly.
  • It takes more time to get the engine to the right temperature.
  • Poor heater performance

How Much Does It Cost to Fix the Duramax P0128 Code?

It is possible to fix the P0128 code at home by yourself, but not always. The results would be good, though.

If you fix it at home, the thermostat may cost $15-300. At the same time, an ECT sensor might cost between $300 and $320.

The cost of labor to change a thermostat is between $150 and $300, and an ECT sensor costs about $100.

Can you drive with a P0128 code on your Duramax?

Even if you drive the car with the Duramax engine for a long time, there won’t be any major problems.

If the temperature is between 170 and 180 F, it’s fine. But once it drops below 150 F, you should take it seriously because it could damage the engine.


In conclusion, understanding what triggers a diagnostic trouble code like a P0128 can help you identify and repair problems with your Duramax engine quickly so you can get back on the road without issue.

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