Engine Troubleshoot

What Are The Symptoms Of A Failing Engine Control Unit?

symptoms of a bad engine control module

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The engine control unit (ECU) is probably one of the most important components in your car or truck. It is responsible for linking together all of the control modules and sensors in your vehicle and making sense of what they are monitoring

The symptoms of a bad engine control unit include a check engine light displayed on the dashboard, a vehicle that is 'dead' or won't start, a failure of one or more of the vehicles systems and issues with the performance of the engine (if it starts).

The engine control unit (also called the engine control module or powertrain control module) is responsible for controlling all aspects of the engine performance when it is running. If it is failing then you are going to experience problems with the engine and how well it operates.

A failing engine control module can also affect the many other electronic systems. These systems and sensors need to be able to communicate with the ECU to operate. If the ECU is failing you may also experience problems with the ABS brake system, the HVAC (air conditioning), the door locks, electric windows, the vehicle lights and even the in-car entertainment system. 

In this article I'll run through the most common symptoms of a failing engine control unit and explain what's involved in replacing it.

The Symptoms Of A bad engine control Module

The engine control module is responsible for monitoring the numerous sensors connected to your vehicle. Its job is to tweak the performance of the engine using the values from these sensors. If a sensor reports a value that is out of the normal range or does not relay any signal, well this usually means there is a problem with the sensor in question. 

But how do we know if there is a problem with the engine control module itself? This can sometimes be difficult to answer, as the symptoms of a faulty ECU can often mimic those of a faulty engine sensor. Sometimes it can take a lot of troubleshooting and sensor testing to diagnose a bad ECU.

Sometimes you can get lucky and it can be obvious that the ECU is to blame. An ECU can develop an internal error but it may still be communicating with the rest of the vehicle.  If you run a diagnostic of the ECU to check for error codes, the code P0606 will sometimes show up. This means that the ECU has detected an internal error while running a self check. 

There are other symptoms of a bad ECU, here are a few of them:

1. Check Engine Light Is On

The check engine light can come on for lots of different reasons. It can be the first indication that there is something wrong, especially if there are no other symptoms.

Sometimes the check engine light will come on and there will be nothing wrong with any of the sensors or systems in the vehicle. This may indicate a fault with the engine control unit. 

In this scenario it's best to start with an engine code reader. Check for any stored codes and keep note of what they are. Clear all of the engine codes and take the vehicle for a drive. If the codes come back then you will need to investigate each one and their associated sensor or system for problems.

If you can't find any problems and everything is running ok then the engine control module can be to blame. If there are random error codes popping that differ every time you clear them, this can also point to a faulty ECU.

2. Poor Fuel Economy

The engine control unit is in charge of the fuel delivery rate during the combustion process. It calculates the necessary rate of fuel delivery to the injectors by monitoring the current driving conditions such as current engine load, the position of the throttle and the amount of being sucked into the manifold.

If one or more of these values are not picked up by the ECU, then it won't be able to run the engine as efficiently as it should. This will lead to an overall decrease in performance even though it is actually using too much fuel. 

Poor fuel economy can be a symptom of lots of different engine component failures. It can be difficult to pinpoint the ECU as being the source of the problem, and it often only comes to light that the ECU was in fact the problem after it has been replaced (for other reasons). 

3. Intermittent Engine Problems

The ECM controls the running of the engine. If it is experiencing problems, then this is going to cause engine problems such as misfires, surging and stalling.

The reason you may experience problems is because the ECU is controlling the fuel delivery by assessing the air intake, the throttle position, the camshaft and crankshaft positions and many other engine variables in real time. A faulty ECU may be loosing its connection intermittently to one or more of these sensors, causing it to behave erratically.

This particular symptom can be difficult to figure out correctly. In reality you (or your mechanic) are going to have to check all of the main engine sensors one by one to rule them out. If the engine is starting and running it may be helpful to get someone to take it for a drive while you monitor the engine in real time using a diagnostic scanner. This can usually help to pinpoint the parts of the engine that appear to be running properly (such as the fuel delivery, throttle position, intake manifold pressure etc ) but may not be communicating with the ECU at all times. 

4. lack of engine power

If the ECU is not doing its job then the engine will perform badly. This usually translates to a lack of power and acceleration when needed.

This can be caused by a number of things. At a basic level, if the ECU is not effectively monitoring the current engine conditions then it cannot tweak the fuel delivery to match the air intake. It may also stop communicating with the transmission control module, and this can lead to problems with being in the wrong gear when trying to accelerate or decelerate. 

The ECU also monitors the engine cooling system. If it detects an engine overheating then it can active the auxiliary radiator fan. It is also involved in controlling the electronic thermostats in modern engines. These thermostats can be opened and closed by the ECU to increase engine combustion efficiency and to reduce emissions.

5. The car is 'dead'

Sometimes if the ECU fails completely it can leave you with a vehicle that you'll be unable to unlock. In most modern vehicles, the ECU links up with the body control module (BCM). 

The BCM is responsible for controlling the central locking, the electric windows, the external lighting and other systems not related to the engine. It often works independently of the ECU, but sometimes it will communicate with it if it, for example when starting the engine. 

For this reason, if the ECU is faulty it can cause problems with the door locks, the immobilizer and the ignition. 

how to test an engine control module

It can be difficult to properly test an engine control module if you suspect that there are problems with it. To figure out what's going wrong, you will need specialised specialised tools such as circuit testers, diagnostic equipment and vehicle specific software. 

There are lots of PCM test and repair facilities that can test your ECM for you, and even reprogram or repair it for you. This type of service can work out much cheaper than fitting a new ECM, especially if your vehicle is an older model. You can expect to pay around $200-$300 for a repair service or for a refurbished ECM.

To help diagnose if it is in fact the ECM that is causing the problem, there are a few things you can do before you replace it:

1. Perform a visual inspection of the ECM

Check for water damage or visual signs of overheating such as a melted plastic casing or a smell of burning. An ECM can often become faulty due to a burnt out circuit board or a short in the electrical wiring.

2. Run a diagnostic test using a code reader

Try to connect to the ECU using a diagnostic code reader. If the reader can't communicate with the ECU then you may have a problem. If it can connect, then try to retrieve any error codes that are stored in memory. Error code P0606 can sometimes show up, and this means that there is a problem with the ECU itself.

3. Investigate any error codes stored in the ECU

If there are error codes in the ECU memory that suggest a problem with the engine or one of its sensors, check each one thoroughly to see if they are faults or false positives. 

Most of the time, if an error is caused by a faulty component, then there will be symptoms to accompany the error. If the symptoms can be attributed to the error code then try to rectify them, clear the error and see if it comes back after driving the vehicle.

4. Check the vehicle battery

A bad car battery can create electrical gremlins in your cars electrical systems. The engine control module needs a steady supply of power and the correct voltage to operate properly. If the car battery is dying, or some of its cells have failed, this can cause the ECM to behave strangely.

Testing the battery is fairly straightforward using a battery tester or multimeter. Check the alternator too, as this can also cause problems if it's faulty. If necessary, replace the battery and alternator and clear the ECM memory of any errors. If the errors come back, it may be the ECM that is to blame.

How much does it cost to replace the engine control module?

Replacing the engine control module can be expensive, especially if you drive a newer vehicle that is not under warranty. 

If your car is still under warranty then you should have no problem getting a new ECU fitted for free. If your car is not covered by warranty, then you have a few options.

The first is to take your car to a dealership and get them to fit a new ECU. This is the most expensive option and is likely to cost in excess of $1500, depending on the vehicle make and model.

The second option is to take your vehicle to an engine control module specialist and get them to fit a new or refurbished ECM. This can be considerably cheaper, and can cost anywhere from $200-$500. Most of the cost is for the labor involved in programming the ECM, and then fitting it to the car. This is probably the option I would choose as these guys can test the ECM when it has been fitted and tweak any settings if necessary. They usually offer a warranty too, so that you can go back if there are any issues down the road.

The third option is to send off your ECM for repair and reprogramming. This is often the cheapest option. Usually this type of service has a fast turnaround time as you are actually going to get someone else's refurbished ECU back in exchange for your broken one. It can also be the cheapest option with prices as low as $150 for older vehicles.

The downside of this option however is that you must remove and refit the ECM yourself.You also miss out on having the ECM tested in your vehicle, so it may not perform as well as it should, even if everything appears to be ok. You will also lose any custom coding that you may have added to your old ECU, as these modules are off the shelf with stock manufacturer software installed.

About the author

Matt Taylor

Matt Taylor

Hey, I'm Matt, founder of themotorguy.com

I've been a car fanatic all of my life. As a youngster, I loved nothing more than taking things apart and rebuilding them. I would also spend my weekends helping my dad with all sorts of car maintenance jobs.
For the past decade I've been a keen amateur mechanic with a love of classic BMW's. Over the years I've learned lots about car maintenance, car detailing and troubleshooting common engine problems.
It's my goal to share my knowledge with fellow amateur car fanatics around the world, and to help people to diagnose and fix their own cars.
If you have any questions please contact me, I'd love to hear your feedback and suggestions!