FixMyCar explains what an ECU is, what it does, and how much it costs to repair.
Book a car repair nowEllie Dyer-Brown
Monday August 11 2025
4 min read
The engine control unit (ECU) keeps the engine working smoothly. It’s an important component and one that it’s worth learning more about.
Your car’s ECU controls most of the electrical systems, from delivering the correct amount of fuel and ensuring combustion happens at the right time to sending and receiving signals about how much you want to accelerate. Any number of faulty wires, switches or components can cause an ECU to flash with a fault. If you have a warning light on, I would suggest getting a diagnostic test to determine the fault and avoid a mechanical breakdown.
Matt Wrankmore
Head of FixMyCar's Garage Network
The ECU is a small computer that manages engine performance using sensors. It controls all functions within the engine, including braking power, ignition timing, the air/fuel mixture and wheel speed.
Sensors throughout the car engine feed into the ECU, which processes the information they provide. It makes millions of calculations every second, comparing the data to performance maps, which outline the ideal operating conditions for the vehicle.
The ECU processes readings provided by the sensors and makes adjustments accordingly. For example, as you are driving, it can detect whether the engine is running too lean (not enough fuel) or too rich (too much fuel) and adjust the balance in a split second to improve efficiency.
If the ECU detects a problem, it will make the driver aware by switching on a warning light.
The location of the ECU varies between different makes and models; there's no industry standard. It will be near the engine - often in the engine bay or the front of the interior (e.g. in the glove box). Your vehicle handbook should provide information about where to find it.
Each car has an ECU designed, calibrated and mapped precisely for its engine. They aren’t interchangeable; the optimal parameters for one car might make another extremely inefficient and, more likely, won’t work.
The ECU pictured below is for a car with an automatic transmission.
If you have to replace your ECU, you should always use an Original Equipment Manufacturer (OEM) part.
ECU remapping involves overwriting the existing performance and software maps with new ones to alter the car’s behaviour. Remapping - sometimes called chipping or chip tuning - is an aftermarket modification popular with drivers who want extra performance from their engines.
This process can be done with a computer or a handheld device that plugs into the Onboard Diagnostic Port (OBD). However, it isn’t straightforward and should only be carried out by a professional.
Below are the pros and cons of remapping. A remap can be tailored to deliver different results, so the outcome depends on which performance areas you decide to prioritise.
Increase engine torque for faster acceleration.
Reduce fuel consumption by up to 10%.
Unlock up to 30% more power.
May reduce the lifespan of the engine and its components.
Invalidates your manufacturer’s warranty.
Your insurance premium could go up; you have to inform your insurer.
Remapping typically takes an hour or two and costs between £150-500.
The ECU is a sophisticated computer, so it’s hardly surprising that it sometimes fails. The symptoms vary significantly but may include:
Problems in the ECU can have serious consequences. For instance, the engine could overheat or its emissions could skyrocket, potentially leading to an MOT failure.
A damaged ECU can often be repaired with the right tools. It involves a lot of testing and specialist equipment to identify the fault and carry out the detailed electronics work required to repair it.
Issues with the ECU are sometimes caused by faulty sensors or wiring rather than the unit itself, which is much easier to fix.
Modern cars can't operate without an ECU because it controls vital systems, including fuel injection. However, they may be able to work with a faulty or malfunctioning ECU; it depends on the severity of the problem.
The average cost of an ECU repair in the UK is £170.87 when you book through FixMyCar.
| Car Make | Average Price |
|---|---|
| Audi | £171.87 |
| BMW | £149.32 |
| Citroen | £194.10 |
| Ford | £187.56 |
| MINI | £121.83 |
| Mercedes | £165.07 |
| Peugeot | £181.79 |
| Renault | £134.32 |
| Vauxhall | £150 |
| Volkswagen | £179.03 |
| Volvo | £104.32 |
Want to find out if your car's engine control unit is faulty? Book an inspection with a great local garage using FixMyCar. Enter your vehicle registration, and you'll receive quotes from mechanics in your local area.
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Ellie is FixMyCar's Content Manager. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers that is easy understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.
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