5 Causes of Engine ECU Failure (Why Does an ECM Go Bad?)

(Updated on June 30, 2020)

The ECU is the engine control unit of your vehicle. It is also often referred to as the PCM (powertrain control module) or ECM (engine control module).

This electronic module is a built-in computer which your vehicle depends on for managing a variety of its systems and functions. Think of it as the vehicle’s brain. The engine and drivetrain are the most dependent on the ECU.

Through a numerous array of electronic sensors, chips, and components within the vehicle, the ECU can transmit data feedback to the engine. The nature of this feedback will determine which function the engine will perform next.

This helps a great deal with calculating the proper amount of air and fuel needed for the internal combustion process. That way, better fuel efficiency can be achieved.

Common Reasons for ECU Problems

The ECU regulates virtually every important system and function in your vehicle. If you were to ever have a bad ECU, then many noticeable symptoms would present themselves. It wouldn’t take you long to realize the ECU was responsible.

However, what you may wonder about is why the ECU malfunctioned in the first place. There are lots of internal causes which could contribute toward this. Below are the top 5 causes of a bad ECU. Investigate them as soon as you notice problems with the ECU.

Read also: Car Immobilizer: Basic Function and Bad Symptoms

1) Dead Battery

old car battery

Car batteries have electronic cells which need to function in order for the ECU to function. If any of the cells in your car battery are dead, then your ECU is going to fail soon afterward. Once all the cells are dead, the battery is considered dead and your entire vehicle won’t be able to function.

You won’t be able to even start your vehicle and get the engine running. So, look out for the early warning signs of a malfunctioning ECU and then have your battery examined right away.

2) Corrosion

corrosion - water damage

The ECU has seals around it which are supposed to prevent moisture from entering inside. However, after a number of years, these seals tend to become worn out. If the seals become too worn, it will be easier for moisture to pass through them and enter the ECU.

Moisture is a bad thing to have in an ECU because corrosion will form on its components. If the corrosion is not cleaned away quickly, it will cause these components to get damaged. Then you will have a malfunctioning ECU.

Related: How to Clean Corroded Battery Terminals

3) Low Voltage

car wiring low voltage

The average ECU is expected to have at least 9 volts, but preferably 12 volts. There is wiring that runs into the harness of the ECU where you can check the voltage amount.

All you need to do is connect a voltmeter gauge to it. This device will be able to detect how much voltage is running through the ECU. If the voltage is 6 or less, then this is likely causing your ECU problems.

4) Bad Jump Starting

dead car battery

Whenever a jump start is performed on your battery, you need to ensure that the jumper cables are attached properly. If you attempt to jump start your vehicle with the cables attached incorrectly, then it could short out your ECU by causing it to spike too quickly.

This will require you to get the ECU repaired or possibly even replaced. This can end up costing thousands of dollars in expenses.

5) Bad Starter

bad starter motor

Many vehicles have starters with their own sensors in them. One of these sensors is the override sensor which manages how many volts the ECU will receive.

If this sensor were to go bad, then the ECU would not receive the correct amount of voltage. Then it would malfunction and cause further problems in your vehicle.

Another problem that could happen is if you replace your starter motor with a new starter that is not compatible with your vehicle’s ECU. Make sure any new starter that you purchase has the right sensors which can communicate with the ECU properly.

79 thoughts on “5 Causes of Engine ECU Failure (Why Does an ECM Go Bad?)”

  1. Hi I think my EC may be bad… If my van was used to JumpStart another and was incorrectly connected, would the ECM stop working immediately or would it take a couple of times that it ran before I could not start my van.. it was jumped sat a couple of days then I drove it about 20 miles, turned off, got back in a few hours later drove probably another 15 miles turned it off 2 days later it cranks but wont start… I pulled over codes and I get a bad reading on just about every components attached to the ECM

  2. Hello,
    i would like to make a question,i tried to connect a diesel blow off valve on a ford fiesta 1.4 diesel 2011 kjva engine code, i did a mistake and i connected the signal wire that blow off valve operates to the signal wire of the throttle position sensor after that i have malfunction to my ecu, can you tell me if with my action i did something to cause the issue? (when i open the board of the blow off valve i noticed a bridging between the pins)

  3. Hi , my ABS/Traction control warning lights came on on dashboard! Scan tool said ecu internal fault! Any suggestions would help me so much?

  4. Hello!
    We bought a freelander 2009 and before it’s left the forecourt they’ve got someone working on the ECU. Apparently it’s not talking to the gearbox … but ECU gearbox is fine.
    If they manage to rectify it should we accept the car or leave well clear …
    Any opinions most welcome. Thanks!

  5. I have a 1999 ford f150. My ignition cylinder lock broke. My car wouldn’t turn off so I pulled the fuse to the ignition switch to get the truck to shut off. Could that cause the ECM to fail? I changed the ignition cylinder lock and then programmed the key. The new key wouldn’t program properly. I called a locksmith to program my key. Once he arrived he tried programming the key which failed. So he then replaced the security relay antenna as well as my dash cluster. Which still wouldn’t program or start. Then he mentioned the ECM may be fried or damaged. So I am wondering could the ECM be the problem or is it something else?

  6. When testing my connector from my ECU to my throttle control sensor I am getting a high volt number on one terminal. On the 3 terminals I am getting .5 volts on the bottom and middle prong inserts, but the top reads a high 78.4 volts. I this a sign my ECU is bad?

    • 78.4 is quite high, but I wouldn’t be so quick to blame the ECU. It sounds like you may have an electrical short somewhere, and you’ll probably have to do some in depth electrical diagnostics to find the root cause. Consult your vehicle’s factory service manual to confirm the expected voltages on your throttle position sensor pins, and work backwards from there to the ECU. You can also test the pin on the ECU to see what voltage is expected and what is actually produced for the throttle position.


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