5 Symptoms of a Bad ECU (and Replacement Cost)

Last Updated on July 24, 2020

The engine control unit (ECU), also referred to as the engine control module (ECM), is an electronic module that controls the function of the engine. This is done through a series of electronic components and sensors which compute data back to the engine.

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Based on what this data indicates, the engine will perform a specific function. If your vehicle has a bad engine control unit, then it can cause all kinds of problems with how the engine functions. As a result, it will impair your ability to operate your vehicle safely and effectively on the road.

Related:  Symptoms of a Bad Powertrain Control Module (PCM)

Top 5 Bad ECU Symptoms

Fortunately, there are certain symptoms that you can look out for early on to detect if your engine control unit is going bad.

#1 – Check Engine Light

check engine light

The easiest symptom to recognize is when the “Check Engine” warning light illuminates on the dashboard. Of course, there are many possible reasons for why this warning light would illuminate.

But in cases where the engine control unit detects issues with its electronic components, circuits, and/or sensors, it will immediately cause the Check Engine warning light to illuminate.

Sometimes the engine control unit will do this by mistake too. To figure out if the engine control unit is truly having problems, you need to search for the specific trouble codes on your vehicle’s computer.

These codes will identify the root of the problem. Check with your manufacturer, owner’s manual, or various online resources to learn more about trouble codes.

#2 – Engine Misfires or Stalls

cylinder misfire causes

Another symptom that may follow a warning light illumination is an engine that misfires or maybe even stalls. There won’t really be any consistency with these issues either. The pattern of engine misfires and stalls will be random, making it hard to pinpoint the specific engine problem.

Often times, this is because the engine control module is failing and it’s feeding the engine some false information. If the problem gets worse, the performance of the engine will deteriorate while it’s running.

#3 – Bad Gas Mileage

poor gas mileage

Since a proper air-fuel ratio is needed for a vehicle to function optimally, a bad ECM won’t understand how much fuel should be sent to the combustion chamber. This means too much fuel is often sent to burn in the combustion process.

The result is that your vehicle will then be running rich and using more fuel than necessary. Not only will you have to budget more for gasoline each month, you won’t be doing the atmosphere any favors. 

#4 – Poor Engine Performance

brake noise while driving

Alternatively, a faulty ECM may also cause not enough fuel to be delivered to the combustion chamber. This means that when you step on the gas, your car simply won’t accelerate like it used to.

It may even shake and vibrate some, especially when trying to go up hills, when there’s a lot of extra weight in the car, or when you’re towing something. You may also experience a rough idle due to not having enough fuel for the conditions.

#5 – Car Won’t Start

car won't start

The worst symptom is when your car does not start at all. It may be difficult to start at first but then, it will never start up again. This happens after the engine control unit has a complete failure and does not function at all.

Once that happens, the engine has no computer system to control and guide its own functionality. Although you may still be able to crank the engine, it will not start because the computer is not feeding it the important information it needs to perform its job.

Again, there could be other reasons for your engine not starting up too. The only true way to know if this symptom is related to a failing engine control unit is to have it inspected by a certified auto technician.

ECU Replacement Cost

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ECU replacement cost

If it is discovered that you truly do have a bad engine control module, then you will have no choice but to purchase a replacement unit or possibly repair it. The cost of an engine control unit replacement will depend heavily on the make and model of your vehicle.

The more complex and technologically advanced a vehicle is, the more you will likely pay. ECUs for luxury vehicle brands will cost much more than that of a typical economy car. It’s similar to buying a top of the line MacBook Pro from Apple versus a budget computer from Walmart.

On average, you can expect the parts cost to be anywhere from $400 to $1,400 while the labor costs are roughly from $100 to $200 for reprogramming and installation. You may be able to save some money on a refurbished ECU but that usually comes with a bit of risk.

The average owner of an economy car will end up spending close to $1,000 for a replacement job. In some circumstances, an existing warranty on the vehicle may cover this expense if the engine control unit fails prematurely.

Sometimes you may have a problem with the engine control unit that can simply be repaired. This will certainly save you the huge expense of having to pay for an entirely new unit.

However, in most cases, the unit will have to be replaced. The only real time when a repair job will be necessary is if there’s merely a configuration problem with the computer.



  1. My ECU is not working well. A.C does not function. Garage stated they called Germany and Mercedez said, “Vell we’re sorry we don’t make that ECM anymore.” Vehicle is a 2002 500sl. Any idea what’s the best avenue? Please let me know.

    • There should be plenty of ECUs for a 2002 MB available. Your local dealership should be able to get a new one ($$$) but many companies sell remanufactured ones, eBay sells used ones, and many wrecking yards (some are MB specific) should be able to sell you one. You will likely need to also have it reprogrammed for your vehicle but again, many online companies offer this service as well.

  2. i have a problem of toyota rav4 2.2ad 4wd diesel engine, the car cranked but not starting i dont know the problem. i diagnosed it the code says C1210 and C1280 both said the Engine control system malfunction.

  3. Will it damage the ECU if starting the engine with other switches at ON position like the Air-condition, rear screen heater, radio etc or will it be better with all the other switches at OFF position when starting the engine?
    Thank you

    • No, it’s perfectly fine to start the car with the HVAC and radio switches in any position.

      If your battery is low on charge, the vehicle may have a bit more trouble starting, especially if it’s been sitting for a while or you’re in a colder climate. The extra load may cause the starter to crank more slowly, or not at all. However, you shouldn’t have to worry about this at all with a healthy battery.

  4. While driving, my doors were unlocking and locking then dashboard lights came on and off several times…vehicle did not stall but in fear I pulled over. This occurred 3 times within a half hour. I prayed and 2 hours later made it home without incident. This was yesterday…I live hour from anything. I’m stumped…module. Electrical, battery…this is a Chevy truck

    • It sounds like you have a short somewhere in the electrical system. If this is the case, it will be almost impossible to pinpoint the root cause without putting a multimeter on the truck.

      If you like to DIY, you can try grabbing a multimeter and a repair manual with a wiring diagram, then start checking different circuits to make sure you’re getting reasonable numbers.

      It’s also possible you have a bad ground. You could try checking all ground straps to make sure they are clean and tight. Make sure there is no corrosion on the ground straps or the battery terminals, as corrosion inhibits the flow of electricity.


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