Home Diagnosis and TroubleshootingElectrical System and Electronics ECU Car – What Is An ECU And Why It Is So Important?

ECU Car – What Is An ECU And Why It Is So Important?

by Jordan Harris

Have you started experiencing some weird symptoms with your car, such as the check engine light appearing for no particular reason and other similar things? Well, maybe it’s because of a bad ECU car. If you suspect that, then you are at the right place because there will be a lot to cover on this topic where we will elaborate everything in depth.

Having a problem like this is something that nobody wants. When the ECU fails there will be a ton of problems in your car that will practically make it unusable and very unpleasant to drive on a day-to-day basis. And if it completely fails, you will not be able to drive the car at all.

This is why you need to start learning more about cars and introduce yourself to car diagnostics. Knowing how to diagnose stuff will be extra beneficial for you in the long run. Why pay a ton of money to someone else to do a thing that you can do by yourself?

This is why you need to step up your game and learn more about how this diagnostic is done the right way so you can diagnose a bad ECU car in a fraction of a minute. And we are going to help you out with that.

First, we will cover what is ECU car is and learn more about the basics. Then we will cover the location of this component and after that, we will discuss the symptoms and the diagnostic process for this component. Lastly, we will cover something more about the cost of this component. So, if you want to learn more, follow along.

What Is ECU Car And What It Means?

Now before we dive into more complex topics like the ECU location, let’s first learn the basics and cover what is ECU car in general? Knowing this will help beginners who are not into cars and want to learn more about this component and what are its main roles when it comes to running the car. So, let’s begin covering the basics.

ECU car in simple words is the car computer, the brain of the car, and this brain basically makes all the complex decisions to make sure that everything on your car works in perfect condition.

The ECU abbreviation stands for Engine Control Unit. This means that this component controls everything when it comes to the powertrain.

This computer is constantly observing the components. Namely, the different sensors that are placed in different places inside the engine bay.

Then these sensors are sending data and other important information to this ECU car unit and then based on this data, the ECU determines what to do.

A simple example is adjusting the air to fuel mixture. Based on the information from the MAF sensor and the O2 sensor the ECU is adjusting the air to fuel mixture in the car and makes the engine work correctly. There are a ton of similar examples to this one.

But what is important for you is that this ECU runs on a map. This map is predetermined from the factory and properly tuned so the engine delivers the best result possible when it comes to performance and fuel economy, this also helps the engine longevity since the engine is working at safe settings all the time. And you can alter these settings if you do an aftermarket tuning. More on that later on.

ECU Location

Now as we covered the basics of the ECU car, let’s now focus more on the ECU location and learn where this ECU is located in the car?

And the answer to this question really depends on the car manufacturer and the model of the car.

Carmakers in order to save some space sometimes decide to tuck these components in places that are really hard to reach.

There are tons of different possible locations for the ECU in a car. Some carmakers install these components inside of the cabin. They place these components either behind the glove compartment or above the driver’s pedals.

Some carmakers install these engine control units in the engine bay. In the engine bay, they are usually located at the passenger’s side close to the firewall. But this isn’t always the case and some car makers install them close to the driver’s side firewall.

Some carmakers like GM place the ECU on the Corvette behind the fender. This means that you will have to remove the fender to reach for this component.

For the best results when you are looking for this information. Namely the ECU location. It is advised to go online and browse your make, model, year of production + “ECU location”.

By doing this you will get the most precise results possible and you will know where to look straight away. This will save you a ton of time, not to mention that you will avoid removing plastic panels trying to find this ECU car component. But if you didn’t know, the ECU is not always called ECU. Sometimes is called under different names? So, what are these names? Let’s find out.

Are PCM/ECM The Same As ECU?

Now let’s answer one question that bothers a lot of people around here. And that is if PCM or ECM is the same as ECU car?

The short answer to this question is yes, they are practically the same thing. So, you should never get confused when you hear one of these abbreviations. But to simplify things we are going to further explain the meaning of these abbreviations.

As we learned, the ECU abbreviation stands for Engine Control Unit, which is the car computer in simple words. But what does ECM or PCM mean?

Well, ECM stands for Engine Control Module and it is mostly used by Ford and GM. This is actually the ECU of the car or engine control unit.

But the PCM is somewhat different. The PCM stands for Powertrain Control Module. This means that this unit is in charge of the whole powertrain which includes the engine and transmission. So, the ECU and the TCM which is the transmission control unit are included in this component called PCM.

We can conclude that the PCM is a much broader component that serves more purposes besides the engine. This PCM abbreviation is used mainly by GM and Ford.

Some carmakers are also calling this component the Injection Control Unit. Some such as BMW also call it DME shortened for Digital Motor Electronics.

The way not to confuse yourself is to look online for your make and model and try to add the abbreviations ECM, PCM, ECU car and see for which one you will get the most useful information.

In the next chapter, we are going to cover precisely where we will learn why this ECU car fails and after that, we will go through the bad ECM symptoms, so follow along.

Reasons Why The ECU Car Fails?

Now before we cover the bad ECM symptoms, let’s learn more about the reasons why the ECU car fails? What are the main causes for this car computer to fail and cause a ton of issues on your car? Let’s elaborate.

The ECU car is a heavy-duty component that is built to last for years. But it has one downside, it’s an electronic component. And as we all know, electronics break very often. So, what could cause this component to fail?

The prime cause for this component to fail is age. The older the ECU, the more problems that you will have. The electronics inside will start to malfunction and fail completely in some cases.

This is very noticeable in the 80s, 90s, and even in early 00s cars where these computers fail. The older the car is, the bigger the chance for this component to fail and develop bad ECM symptoms.


The second probability is damage from water and moisture. If your car was flooded, then it is highly likely that you will have a problem with the ECU of this car.

Electronics do not like water and moisture. So, the internals of the wiring harness and the ECU have to be replaced if they get wet. So, if you look for a flood car at an auction that does not have a signal. Stay away from it because the repairs will be crazy expensive.

Imagine that you need to replace the whole harness and the ECU, it could easily cost you $1,000 or more depending on the model that you purchase.

This is why you need to beware of these things. The newer the car the less chance you will have to end up with a bad ECU car.

Bad ECM Symptoms

As we cleared the basics of what is an ECU car, now we can focus on other topics, namely the bad ECM symptoms. As we concluded ECU and ECM are basically the same things. But ECM is more frequently used so we decided to call this chapter like that.

Nevertheless, knowing the bad ECM symptoms will help you when it comes to diagnosing a bad ECU car and will tell you if you need to replace the computer of your car. So, what are these symptoms? Let’s elaborate on them in the next chapters.

1. Frequent Check Engine Lights

The first on our list of bad ECM symptoms is the check engine light. Whenever this ECU car is broken, you will get more and more frequent check engine lights on the dash.

At first, you will think that there could be a problem with the car and will dive into diagnosing the problems. But you will come to the realization that there will be no issues with the sensors and other equipment.


Sometimes when this ECU car fails, it will start to go into what I call, the Christmas tree mode. This is the situation when all the lights turn on the dash and you are completely freaked out that your car is totaled.

But this isn’t the case in reality. All these issues can be attributed to a bad engine control module and sorted out if you decide to replace it with a new component.

This is why whenever you get some really illogical issues with components, you should consider the ECU as one of the causes of this problem. Replacing this component with a new unit or a used one will sort out the problem with your car and hopefully will not repeat itself in the near future. Now let’s move to the next bad ECM symptoms.

2. Engine Misfires

The second in our list of bad ECM symptoms is the situation when you are experiencing misfires in your engine and this is happening really frequently. So, why is this the case with your car?

Well, this is the case because the ECU car is not working correctly. This means that this component is malfunctioning and not making good decisions when it comes to the proper engine work.

It will fail to adjust the air to fuel ratio and could also cause other components such as the engine coils, and injectors to malfunction and basically bug down. Like a broken computer usually does. So, when this types of situation happen and you think that everything is in good condition. You better check the ECU car and see if it’s working well. If it doesn’t you will have to replace it.

3. Poor Fuel Economy

The third on our list of bad ECM symptoms that we would like to cover is the poor fuel economy. Along with the misfires, this symptom will be also present.

The computer will fail to adjust the air to fuel mixture in the car and will cause the car to malfunction.


The computer will tell the engine to dump too much fuel in some cases, resulting in rich air to fuel mixture. You will also be able to experience the smell of gasoline from the exhaust when this situation happens.

That’s why when you notice something like this and every component is working well in your car, it is worth checking the ECU car and seeing its condition of it. If it’s bad, then replace it with a new component.

4. Bad Engine Performance

The third in our list of bad ECM symptoms is the situation when you have bad engine performance. In the previous chapters, we have covered the engine misfires along with the increased fuel consumption and in this one, we will cover the other engine problems.

Namely, whenever you have a problem with this component, you will highly likely are going to face a situation such as poor engine work.

By poor engine work, we mean misfires and situations when the engine is significantly down on power. With a bad ECU car, you can lose up to 30hp or more from the performance of your engine.

In addition to this, you will frequently have problems with your car stalling in traffic. The car will stall because it will not be able to adjust the air to fuel mixture and this will greatly affect the performance of the engine. Whenever you face problems like this for no particular reason. It is worth checking the ECU and seeing if it’s working well, if it’s not, just replace it with a new one.

5. Car Doesn’t Want To Start

And the last in our list of bad ECM symptoms is the situation when the engine does not want to start and the engine is completely dead.

This is the situation when this ECU car or engine control module is completely dead. As we said, this can be caused by a complete failure of this component.


This situation will deem the component inoperable and will prevent the car from starting the engine. When you have a situation like this most often will be the car battery or another ignition component. But if you try to connect a scanner to this ECU you will notice that the scanner will not record anything because this component is not working at all.

But how you can diagnose a bad engine control module? Well, that’s what we are going to cover in the following chapter.

How To Diagnose A Bad ECU Car?

Now let’s see how you can diagnose a bad ECU car. Knowing this will be a great help if you are dealing with a problem like this. There are two situations when it comes to this. When the ECU is faulty and communicates with the car and the situation when the ECU is faulty and doesn’t communicate with the OBD2 scanner.

The situation when the engine control module is bad and the engine keeps working can be tricky to diagnose. Almost impossible, to be honest. Why is this the case? Well, this is the case because you cannot be able to tell if the unit is bad because you get a few codes on the dash.

The only way that you can check in this situation is to get another engine control module and test the car with this new module and see if you keep getting the same codes. Something that can be really tricky to find because you cannot borrow an ECU from someone.

The second type of failure is whenever there is no communication with the engine control module. Meaning that this component is completely dead.

In this case, you will have to check the battery and the ECU with a multimeter and see what type of signal they are producing and make sure that indeed the module is dead and needs to be replaced. Above in the video, you can see the procedure of how this is done the right way.

Or look online for ECU repair near me and take your car over there. I’m sure that there are shops that work precisely with these modules and have a ton in stock and can use them to test if yours is malfunctioning or not.

Cost To Replace The ECU Car

Now let’s cover the cost of replacing the ECU car. How much can you expect to pay in order to replace this component on your vehicle?

If you opt for a new unit, you can expect to pay about $800 on average. Still, for some models, these components can go for about $450. But there are components for some cars that go even up to $2,000. So, you need to be aware of this.

Electrical electronic problems engine control unit

If you want to get this problem on the cheap, you might want to get a used engine control module. Just get one from the junkyard for a fraction of this price. But make sure that the engine and all the harness connectors are the same. Or else, it will not work.

There are also other places online that sell these units used for $300 to $500 for most of the cars.


In this article, we have covered quite a bit when it comes to ECU car. We learned what is this engine control module all about and where it is located. Then we learned the difference between ECU, ECM, and PCM.

After that, we covered the bad ECM symptoms as well as how to diagnose and how much it will cost to replace this component on your car.


Now let’s answer some frequently asked questions.

What Does ECU Stand For

The abbreviation ECU stands for Engine Control Unit. This is basically the control module that regulates the operations of the engine.

What Is An ECU In A Car

The ECU is the car computer. More precisely the module serves to regulate the work of the engine and adjust a lot of processes so the engine works correctly.

Where Is ECU

The ECU can be found in a ton of different places. It could be under the front dashboard or in the engine bay mounted close to the firewall. The mounting position depends from application to application.

How To Reset ECU On Car

You can reset the control module by removing the positive battery terminal and leaving it like that to sit for 5-10 minutes. Then connect everything back in and you will be good to go.

How To Reset Engine Control Module

Just remove the positive battery terminal from its place and leave it like that for 5-10 minutes. Then plug it back in and the computer will be restarted.

How To Tune A Car ECU

Tuning an ECU is a complex process. You will need the software as well as the skill that is required to tune a car.

You may also like

Leave a Comment