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.

106 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.

  7. Mine will run fine a week,then sputters ,nut when in drive it dies, wont run at all no communication with ecu ,then when it runs fine it connects to ecu no codes but speed sensor. 96 lexus we cant find anything wrong with it im mailing my ecu out for repair .

  8. Had a new PCM programed and installed by mechanic, now runs worst than the original that had code p601, what next?Was the programming incorrect?

  9. Engine cranks but does not fire. Fuel gauge does not move. No check engine light. All other dash lights work. Leave switch in on position for 15 seconds check engine light comes on, other lights are off. I replaced the ECM with a rebuilt one ( new module not available for van this old), it worked for 2 starts of engine, then failed.
    1999 Dodge Ram Van 3500, I am the original owner. Mileage 57,500 miles. Now have 2 ECMs that do not work. Auto mechanic worked for 4 hours, could not discover what caused the failures.

  10. I’ve Audi A3 2000 model. One of my cylinders is not working and I’ve been told that my brainbox is the problem after been scanned. I want to buy a new brainbox. Can someone help? How can I?

    • It sounds like you may have a bad connection concerning your cylinder which requires electricity. Your cylinder is connected to a spark plug.
      If I understand correctly it is part of a harness which you can probably replace yourself if you ordered online on Amazon or go to the Pick-A-Part junkyard and get it a new one or used one.
      It would help if you watch you to somewhere and you will find what you are looking for. Good luck

    • Do you have the old one yet if so down load the program vin number and then flash the new one you must have the same part numbers too
      When you get the se one pull the vital info off the old one when you hook the new one up wipe it then reset it with the numbers from the old one in it then make new keys

  11. My car computer box seems to have a fault since the reverse gear is not functioning properly. My mechanic disconnected power to the computer box and the car is running well. If I keep driving the car in this situation, what will be the problem to my car?

    • I don’t know how the car could even start without an ECU. Could you clarify with your mechanic exactly which “computer box” he unplugged?

    • You did not mention if you have an automatic or a standard automobile. Usually if you have a standard it will involve the clutch in all its body flywheel and search and all that stuff.
      If you have an automatic then it will have to do with somewhere in the transmission area.
      Your immediate help would be on YouTube. Good luck

  12. i have a Suzuki grand Vitara 2008 v6 the approximate mileage is 170000 Km, i have an issue with it the car is cracking but not starting i re programed the key then it starts but when i switched the ignition again it returns to cracking and its not starting so tired to program the key again but unfortunately the i couldn’t communicate with the ecu and i check the wiring and fuse box i changed the ecu or bcm but still cant determine the problem

  13. i have Hyundai accent 1.6 2010, i have an issue with it as it is cracking but not starting, it was scanned it codes shows transponder error, key reprogrammed but still did not start, when scanned again after key being programmed engine results are not appearing it only shows a question mark next to engine(?). but when i pour petrol manually in a cuborator it starts but stop after a minute.

  14. Hi
    , I have opel Corsa gamma 1.4 is now giving me problem since I changed a cylinder head it doesn’t wanna start but it cranks then it shows engine check light,, is it a computer box or what?

    • Double check that the vehicle is timed correctly and that all crankshaft/camshaft sensors have been plugged in. What is the check engine light for?

    • Maybe you have a fuel pump issue. A faulty fuel pump can cause problems in your best bet would be to look on you to or ask a friend or a mechanic friend did you can trust hopefully it won’t be something else

      • Type in your problem on YouTube and you’re fine a various response that may help you lead to your issue and getting you some answers good luck

  15. Hi I have a VW Caravelle 2.6i my bus was running i parked it off for 2 weeks the day i decided to use it , it only cranks and does not start we tried everything , it has spark , fuel , checked the distributer and coil fuel pump and even checked if the injectors were spraying out fuel to the plugs and all is working fine , My bus has no alarm , we all suspecting its the ECU cause the guy i bought it from 4 years ago had the simlar problem and he sorted out the ECU .
    I tried different people who knows about busses and they all point out to the computer box .
    What must be the problem with my bus ?

  16. Hey, I have an Elantra 2012 1.6 CC model. The RPM is fixed at 2000 – 25000 with the Engine light ON. After checking the code, I received P0626, P2128, P2138. Funny thing: I have changed the throttle body and the pedal position sensor but cannot make any impact. The strange thing is after even resetting ECU the code is not going and in the end, the technician asked to change the ECU. I would appreciate your comment on this. Also, if I have to replace the ECU, do I have to change anything with it like an immobilizer or key? Or simply changing the ECU with the same ECU model and product number can make an impact? I have to purchase it from the Hyundai dealers and there is no way to return it back to them once used, so I would highly appreciate your help with this. Thanks.

    • Before replacing the ECU, check a service manual to see where the alternator and throttle position sensor share part of the harness. I would check there for broken wires, shorts, missing insulation, etc first before doing anything else. Replacing an ECU is much more expensive than fixing a wiring issue and you want to make sure that’s the root of the problem before trying it.

      Check your grounds while you’re in there. Make sure all grounds are clean and tightened.

  17. Hi my car is Mazda m6 2005model my ecu got damaged and I called for someone to replaced it,but it end up damaging 2ecu,pls what do I do as am afraid of getting another ecu because of the cost

  18. Hi. I have a Opel Zafira A. The ECU is attached to the injection pump in this cars. A few months ago it stoped working. The car wasn’t starting. With the computer we saw it was the injection pump damaged. So we remove the module and sent it to be fixed. After fixed the mechanic mounted it again and the car worked for 20 days more or less. But then it got broken again. Removed the module again , fixed and mounted again. The car worked for 15 days. After this I decided not to repair the module again. I bought a used pump and mounted it on my Zafira. It worked during almost 15 days, but then it stoped working again. The guy who fixed the module in the first 2 times said the module was burnt , so there would be something that would be damaging the module. Now after I installed the used pump I think he might have reason. Do you have any idea about this? Thanks

  19. Hi c180k w203 merc I started the car in the morning it started fine while ldiling for a few minutes I heard it it wanted to cut of and picked up rev immediately there after but I drove for about 30 kms switched off after 20 min tried to start it refused to start pls help


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