Code P0128: What Does It Mean?

Error codes are confusing, which is why we are here to decode them for you and give you simple to follow instructions on what needs to be done to remedy your issue. This particular issue has to do with falling temperatures and an overheated engine. 

Code P0128

Focus shot of man's hand holding a car thermostat.

In the simplest terms, when you see a code P0128 that is an indication that the coolant thermostat temperature has dropped below the regulating temperature. When this device allows the engine to warm quickly the wear and tear on your engine lessens


There is a device in your engine called the engine control module (ECM). This gadget keeps track of the correct operating temperature for your engine and how long it takes for it to be achieved. 

When your engine gets to that temperature, the control module puts the fuel system into an automatic look and the engine attempts to reach the air to fuel ratio of 14.7:1, which is considered the most efficient. It does this by using oxygen sensors. 

The ECM sets the engine coolant temperature sensor for 160˚F within the first fifteen minutes after the engine starts. Once this happens the recorded temperature should not go below that temperature while operating. If this precedent isn’t kept then, the next time you start the car you might get a code P0128. 

Here are some of the signs that this is an issue. 

The temperature gauge is low.
  • The check engine light is on
  • The temperature gauge is low
  • Fuel economy declines
  • The car idles higher than usual

Is This Important?

While any error code is important and worth investigation, this issue is not very serious. The most common cause of this code is a thermostat that gets cemented in the open position. If this is the case you should check your coolant levels before using your car. This could result in the dreaded overheating we spoke of and cause damage to your engine 

The Cost Of Repair

If the issue is replacing some coolant and closing the thermostat then it shouldn’t be too much, but, as with anything in an engine, there could be a more insidious issue so you might need to dig a bit deeper if you find the common problem not to be the issue. 

Getting To The Root Of It

Other reasons why you might get a code P0128 are as follows. 

Hose connecting the car radiator with the engine, In the background the engine compartment.
  • There may be a leak in the engine somewhere
  • The coolant system could be bleeding air
  • Your engine might need new gaskets or a new thermostat

How To Find The Problem

While some codes require a great deal of detective work this one only has a few possible solutions so the investigation process shouldn’t take as long.

Step One

The first step is to always check again to make sure that you have the correct code. You don’t want to start investigating the solution to one problem only to find that it’s another problem altogether. If other codes come up it’s a good idea to take care of those first. 

Step Two

Now take a look at the level of coolant in your engine. Before you add more make sure to check the state of the coolant that’s in there. If it’s filled with rust then that could be clogging your system and causing the P0128. If your coolant is clean then add more if needed. This would also be a good time to make sure there aren’t any leaks. 

Step Three

Cropped view auto mechanic with mud on hands fixing automobile in car repair station

The next step would be to use a multi-meter to check the coolant temperature sensor. As the temperature changes so with the ohm reading. If that doesn’t happen then there is probably something wrong with your coolant temperature sensor or the wiring. You should dig deeper to find out so you can replace or repair what is damaged. 

Step Four

As we said earlier, the cause is most likely an engine coolant thermostat stuck in the open position. This is how you check to see if this is the problem. 

Touch the radiator hose gently to see how hot it is. WARNING!! This can be very hot so do so with caution. The temperature of this house will let you know how fast the hot coolant is heating up the radiator hose. If it takes a while for the hose to heat up then the thermostat is stuck in the open position or can be opened too soon. 


There are a few ways you can fix this code issue. 

  • Check for leaks
  • Bleed your coolant system of air
  • Get new gaskets or thermostat
  • Add coolant

The Cost Of Repair

car mechanic at repair work with thermostat

As with anything else that requires repair the price will depend on parts and how much labor is required. But, with this, the cost could run very low if all you need to do is add some coolant. Now, if all you have to do is replace a few gaskets and a thermostat then the cost will rise. 


Codes are difficult to keep straight, especially when you are new to the whole mechanic thing. That’s why we put together these little pieces to help guide you. Of course, you may be a hobbyist and not feel comfortable going as far as replacing pieces like thermostats and gaskets. 

That’s fine. There is no shame in going to a professional mechanic to do the jobs you aren’t sure comfortable with. Of course, we also like to encourage people to be self-sufficient, so if this is new to you, maybe head over to a pick-n-pull and monkey around with some broken-down engines before working on yours. You might even find some good used parts.  

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