Why Is My Jeep Showing Code P0128? (Meaning, Causes & Fixes)

It’s been a few weeks since that pesky code appeared on your check engine light.

P0128, it says.

So far, you haven’t noticed anything noticeably off about your ride, so it’s easy to brush this issue under the rug.

P0128 Jeep Engine Code

As vehicle owners, even the best of us can make this mistake. Most people don’t know that this issue will affect your vehicle’s fuel economy and adversely impact your engine performance over time. Not to mention the long-term impacts that this will have on your car!

We’re here to help you understand why this code is showing and how you can resolve it.

What Is The Jeep Code P0128?

Jeep Code P0128 means that your Engine Coolant Temperature is below the thermostat regulating temperature. Simply put, the coolant which helps regulate your engine’s temperature is colder than it should be.

What happens as a consequence is that your engine will be running colder.

The ECM, an electronic control unit placed in the engine, detects how long it takes for an engine to reach the correct temperature, usually between 160-200*F

When your engine reaches its correct temperature, it enters into a “closed loop.” This means the oxygen sensors in the engine will create an optimal air-fuel ratio to maximize efficiency.

Standard ECMs detect whether or not the 160* F level is reached within 15 minutes of the engine starting up. If the engine coolant does not reach that temperature within that set time or falls below it at any time during operation, the p0128 code will blink.

What Happens to Your Vehicle When You Have a Code P0128?

A cold engine might not be as catastrophic for your car as an overheated one, but problems will still arise.

First and foremost, you may notice that your Jeep’s heater does not give out as much heat as it used to. This is because the coolant going through the heater will not be as hot as it needs to be. If you notice the heater working better when your vehicle is still, it means that there is an issue with your car’s thermostat.

Another symptom of a code p0128 will be increased fuel consumption in your vehicle. An ideal fuel-to-air mixture will not be achieved, and your car will guzzle more fuel than it needs to. There will not be too large of an increase in fuel consumption, but this is a common symptom of a code p0128.

You may also notice a buildup of sludge in your radiator. This does not happen as a direct consequence of the temperature of the coolant being too low but as an accompanying problem. A leak through the radiator cap may cause oil and coolant to mix, forming sludge. This will result in oil pressure and starting problems. Another nasty consequence of sludge buildup will be blow-by in your engine, which results in damaged piston rings, and snowballs into many more issues, such as low power when accelerating and poor overall engine performance.

What Causes Low Engine Coolant Temperature?

Now that we’ve explored this issue let’s see why it occurs in the first place.

The first and most common cause for a code p0128 in your Jeep will be that the coolant level is too low. Coolant leakages are common and happen for a variety of reasons. Firstly, the cap on your radiator could be worn out. The pressure inside the radiator is extremely high, so when the radiator cap does not do its job correctly, you might end up with coolant leakages. Over time, this will lead to a lower level of coolant.

Another reason could be radiator corrosion, a common issue for vehicles that have been hitting the roads for a long time. Similarly, a broken or worn-down expansion tank can allow coolant to escape through holes or cracks in its plastic surface. 

Another reason you may be facing this issue of your engine coolant being too cold could be that the engine fan could be stuck. Usually, the engine fan only turns on when the coolant runs above a temperature threshold. However, when your coolant temperature sensor is faulty, this fan may keep running even when it’s not supposed to. A short circuit in the wires connected to the fan may cause this issue.

A coolant thermostat stuck in the open position may also be to blame. This causes coolant to run into the radiator continuously, causing the engine to not heat up properly. If the thermostat is stuck wide open instead of just slightly open, you’ll find that this problem will be even worse.

How Can I Fix Code P0128?

How to Fix Jeep Engine Error Codes

Since there are multiple reasons you could be facing a code p0128, there is no linear way to treat this issue.

Generally, a good start is to change the thermostat. The thermostat for most vehicles will generally be cheap and easy to replace. We’d advise you not to do this step yourself as you could risk injury. Heading to the mechanic is much better and won’t cost you much. The exact cost tends to vary greatly between vehicles. 

Next, it’s advisable to check your radiator and the expansion tank. A radiator with visible corrosion will tend to have tiny holes, which may not even be easily visible. Make sure to also check the hoses as well as the hose connections. You can easily have these holes repaired by a mechanic for a small sum.

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While a code P0128 isn’t too alarming, a responsible vehicle owner will not neglect to get to the root of why this issue may be occurring. There are many reasons why this problem occurs, so you must make an effort and find the nitty-gritty of this issue.


Will a Code P0128 clear itself?

Generally speaking, a code p0128 is a sign of an underlying issue with your vehicle’s engine. It will not go away on its own and may even lead to more serious issues if it persists long-term. If a p0128 error is simply caused by cold weather, it may subside as the temperature normalizes.

Can I drive with a Code P0128?

A code p0128 does not require immediate attention, and you may drive safely with this code. It is advisable not to let this issue stay for too long as it would lead to more issues. 

How Long Can I Drive With A Code P0128?

You can drive around 500 miles before a code p0128 is too much of an issue.

Can I fix a P0128 code at home?

While you could attempt to change a thermostat by yourself or even do repairs on your engine parts, it is not advisable for amateurs as the process can be dangerous. It is best left to the professionals to carry out such repair work.

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