How to Fix P0128 Code: “Coolant Thermostat Temperature Below

If you’re searching for how to fix the P0128 code, confirm that the error code exists using an OBDII scanner. After confirming the issue, you need to inspect two components: the ECT sensor or the thermostat itself. If any of these components are faulty, you have to replace them. Otherwise, you need to have a professional mechanic inspect the vehicle for more advanced tests.

In any combustion system, engines should operate within a certain temperature level. While it might sound that temperature is bad for any engine, your engine still needs to reach a certain optimum level to operate properly. When the engine cannot do that, you will notice a check engine light illuminating and throwing an error code of P0128.

To understand what really causes the P0128, this article is purposely for you. We will provide you with a general overview of this error code, along with an explanation of its definition. We will also highlight the primary causes, symptoms, repair options, and rough estimate of repair costs to help you fix a P0128 error code.

What is the P0128 code definition?

The definition of the P0128 code says, “Coolant thermostat temperature below regulating temperature.”

While this definition specifically is more intuitive than others, it is crucial for you as a driver to understand the role of the cooling system and get an idea about how complicated and serious is this error code.

The following section will cover some basic information about the cooling system and the optimum engine temperature to help better you understand this error code.

What’s the role of the coolant thermostat, and how is it linked to the P0128 code?

While it’s known that temperature is one of the biggest enemies to any combustion engine, the engine still needs to reach a certain temperature level to operate properly.

That’s why when you look into your vehicle’s owner’s manual, you will see that engine operating temperature should be within a certain range, which means that if the engine temperature goes below that range, it is still considered bad for your engine.

The primary component in your vehicle that regulates the engine temperature is your cooling system. The thermostat is specifically responsible for allowing the coolant to run through around the engine and bring its temperature down. Sometimes when the thermostat goes bad, the engine will be experienced to the coolant all the time, which means that it can’t reach the optimum temperature.

That’s when your car’s computer would throw a check engine light, and as you scan it, you’ll notice a P0128 code.

When the P012 is triggered, your vehicle’s engine will not work properly, and you’ll notice a reduction in the vehicle’s overall performance along with a significant reduction in the vehicle’s fuel economy.

What are the primary causes for the P0128 code?

When your check engine light illuminates due to a problem related to a P0128 code, the issue is most likely related to one of the following culprits:

  • A stuck opened thermostat
  • A bad coolant temperature sensor
  • Malfunctioning wiring in the cooling system circuit
  • A missing thermostat

How do I know if it’s the P0128 code? Symptoms of the P0128 code

When your vehicle has a problem related to the P0128 code, you can detect it early by watching for the following symptoms:

  • Check engine light illuminating
  • Reduction in fuel economy
  • Much higher idle than normal
  • Very low-temperature gauge

Keep in mind that all mentioned symptoms can be linked to other causes. In other words, you can’t simply say when the temperature gauge is continuously low, the problem is related to a P0128 coat. The only way to confirm and get accurate results is through an OBDII scanner, where you can connect it to the engine and scan for any error codes.

How to fix the P0128 code?

The good news is that the P0128 error code can be resolved at home without needing an expert mechanic, which means that you can save a ton on labor costs.

Our team reviewed all available DIY’s on the Internet as recommended by automotive experts to help you find one designated location to fix the P0128 code at home:

Before we list the different DIYs we’ve reviewed, we need to make sure that we are on the same page confirming the error first and before performing entity otherwise. Many people might suspect that the issue is related to AP0128, but that’s not how it should be done.

Therefore, it won’t take you more than a couple of minutes to just connect the OBDII scanner to your vehicle’s computer and scan the errors.

After scanning the errors, your OBDII scanner will display on the screen the error code number along with its definition. Of course, if there are any other errors in your vehicle, you will see those as well. Keep in mind that some inactive or old errors might be displayed as well; therefore, make sure that you’re looking at the current or active error codes before attempting to fix them.

  • Inspect the ECT sensor

One of the most common issues that might trigger is a faulty ECT sensor. Locate the sensor, which is usually connected to the thermostat. If you had a hard time locating the ECT sensor, you could always refer to your vehicle’s owner’s manual to get better, accurate guidance.

Once locating it, take a closer look at this sensor to see any signs of damages or deterioration. Using a multimeter, connected to the sensor and check if it’s performing within the required level. Usually, if you’re doing the tests in a room of 68 temperature Fahrenheit, the resistance should be somewhere between 2000 and 3000. If that’s not the case, then the ECT sensor must be replaced.

  • Check the thermostat

If you confirm that the ECT sensor is performing just fine, your next step is to take a closer look at the thermostat itself. Locate your thermostat by referring to your vehicle’s owner’s manual and remove it carefully.

Keep in mind that you are not supposed to touch the engine when it’s very hot because the thermostat can be extremely hot, and it can be very dangerous to mess around the coolant. Therefore, once the engine is cooled down, you can go ahead and perform any inspection or replacement to the thermostat.

Inspect this thermostat and see if there are signs of damages or the thermostats not working properly. If that’s the case, you need to purchase a new thermostat and make sure that it’s identical to the old one before installing it.

When installing a new thermostat, it is very important to make sure it’s placed correctly. The thermostat has two different sides: The deep end of the thermostat must be connected to the engine, while the short end must be connected to the hose.

A good tip here is to take a picture of the old thermostat and use it to install the new one. Or you can always have the vehicle’s owner’s manual handy for detailed guidance.

  • Clear the error code

Once resolving the issue, you need to make sure that you manually clear the error code. Any internal error code associated with the check engine light will not be cleared out by itself. You must use an OBDII scanner and ask it to clear any inactive codes.

Once clearing the codes, you can confirm for sure that you resolve the problem. It is also important for you to perform a second scan to ensure that the area code does not appear one more time.

If the error code still exists, your problem was not resolved, and you have to take your vehicle to a professional mechanic. When taking your car to a repair shop, they can take a closer look and see whether you’ve done the DIY correctly or not. They can also perform specific manufacturer tests to determine what exactly is causing the P0128 code, not any of the mentioned possible culprits.

Can I drive my car with a P0128 code?

The P0128 code is not life-threatening, which means that you can still drive your car even if this code is displayed. However, the only situation where you must stop driving this vehicle is when the coolant level is dropping.

Sometimes when the P0128 code is displayed, another problem might be linked to a coolant leak somewhere around the vehicle, and therefore, yours a very high chance that the engine might get self-destructed.

How much does it cost to resolve the P0128 code?

Depending on the cause of the problem, repair costs might be different. For example, if the faulty component triggering a P0128 code is related to a bad coolant temperature sensor, repair costs ranged from $140.00 to $190.

Although hand, if the problem is coming from a bad thermostat and requires replacements, expect to pay somewhere between $220 and $250.

Keep in mind that labor cost is a big component in any repair costs that have to do with the P0128 code. For instance, if you decided to go to a small repair shop, repair costs can be much lower than going into a dealership because of their high hourly rate.

Although that’s the case, many owners of modern vehicles prefer to go to the dealership even it will cost them more just because they are confident that experienced mechanics will be working on their vehicles. There is a very limited chance to introduce additional issues while fixing the car.

Is it worth fixing the P0128 code?

Since fixing a P0128 code requires relatively lower repair costs, fixing it to avoid major troubles down the road is always worth fixing.

The only scenario indicating that it’s not worth fixing a P0128 is when your vehicle has other more complications that are not worth fixing. For example, if you know that your car suffers from major problems in the transmission or the engine, it is never worth putting a couple of $100 to fix a P0128 code.

If that’s the case, you can always reach out to Cash Cars Buyer and sell your vehicle to use the payment towards a better car without any problems.

Can a stuck open thermostat trigger a P0128 code?

That is a great question! Did you know that the thermostat is designed to force it to start open when it goes bad? That’s true because the coolant will continuously keep flowing around the engine, so the engine temperature does not increase extremely high.

So, a stuck open thermostat is a very common cause for triggering the P0128 code.


Your engine must perform within a certain range of temperature, and we always say that temperature is one of the biggest enemies to your engine, but your engine still needs to reach that optimum level to operate properly.

One of the very common error codes that you might experience with engine temperature is The P0128 code, usually related to an issue with the thermostat where the coolant continuously runs around the engine and brings its temperature down.

Although the P0128 code is not as serious as others, it is important for you as a driver to take care of it immediately once you notice it to prevent dealing with major complications down the road.

If you notice that your car has additional problems and it’s not even worth putting some time and effort resulting in a P0128 code, it might be the right time now to evaluate the situation and see whether it’s worth selling your car to Cash Cars Buyer instead.

Cash Cars Buyer is one of the top-rated car removal companies in the nation that guarantees to pay you the top dollars and will provide you with free towing despite your living location around the United States.

The best thing about Cash Cars Buyer is that our process is very straightforward and doesn’t take more than a couple of days to get your vehicle removed safely and for the most money.

To learn more about our process and our team, you can reach out to us by giving us a call at 866-924-4608 or visit our home page click on the free instant online offer.

About The Author