Expert advice: what the ECU does and why it’s so vital for our car

All this is computer controlled. The ECU is normally positioned at the top and back of the engine bay (Picture iStock/jaym-z)

Modern cars are more like computers on wheels and central to that is the ECU. If the engine is the heart of the car, the Electronic Control Unit or ECU is its brain. Your car may develop a fault that you think is mechanical but actually the real culprit could be the electronics, caused by a malfunctioning ECU or one of its sensors.

The ECU is now such a crucial and integral part of our cars that I think it’s worth understanding exactly what it does.

What’s in a name?

You’ve probably heard people talking about Electronic Control Unit, Engine Control Unit, Engine Control Module (ECM) and sometimes even engine management system. These are all the same thing; it’s just some car makers prefer one name over another.

It’s important to understand that the ECU isn’t just one computer. It is a number of systems in a network; some new range-topping cars have up to 80 ECUs. Among other things, these monitor and control the engine, transmission (gears), brakes, electronics, body control and suspension. But to make life simpler, we’ll just refer to it as a single thing.

What does the ECU do?

No matter what you call it, what the ECU does is to take data from a network of sensors. These are placed all over the car and constantly relay a mass of information. Among other things they tell the ECU the air temperature, the temperature of various components inside the car, the angle of the car and its steering wheel, the speed of individual wheels, the position of the accelerator, the gear that’s being used, how much air the engine is taking in and the position of and pressure inside various parts of the engine.

How does it work?

The ECU processes and decodes all the information it receives from this sensor network in real time. It stores some of the data, such as faults, so that technicians can access it at a later date. Other information it will act on. The safety systems are a good example of this should the sensors think the car is out of control and crashing. The fuel injection is another.

The ECU knows how warm the engine is, how fast the car is travelling, and the position of the accelerator and where the crankshaft that controls the valves (and fuel use) is. It can therefore dictate and control the exact fuel-to-air ratio, second by second to maximise performance, or efficiency, or to compromise between the two.

What exactly is the ECU?

We suggest you don’t do this… Like most computers ECUs involve highly delicate circuitry (Picture iStock/Urcisxtrime)

Think of the ECU a bit like a personal computer. It has hardware, essentially a collection of electronic components on a printed circuit board, and software. This enables the ECU to be reprogrammed or updated if glitches are discovered or perhaps a better way to do things is discovered.

Where is the ECU?

The ECU is usually in the engine bay. Its precise location varies from car to car but it’s frequently above and behind the engine, beneath the windscreen wipers. But I’d advise you leave well alone. You probably wouldn’t go poking around in your PC and for similar reasons it’s best to leave your ECU to the professionals.

How expensive are new ECUs?

As we’ve seen, the ECU is an integral part of the engine so if you need to replace one, it’s not going to be cheap. In the unlikely event that you’ll need an entirely new one, it could cost anywhere up to £2000. Happily, most of the time, they can be reprogrammed if they start misbehaving.

There are services that offer to reprogramme ECUs for improved performance or economy. If you decide to have this done, make sure you go with a reputable company. The ECU is such an important component, if someone gets it wrong it could be very costly indeed.

What about the future?

As electronic technology has come on, so has the role of the ECU. And it’s only going to get more important. The self-driving technology that features on many cars now, such as cruise control that maintains a safe distance from the car in front and technology that keeps a car in lane, is all linked to the ECU.

How long has the ECU been around?

Car makers didn’t really start using ECUs to control engine functions until the 1970s. This was when microprocessors and integrated circuits came about. The Japanese started using these in engines in the early 70s. A digital computerised control for fuel injection was first used in 1980 by US company Cadillac. The last major car company to stop selling pre-fuel injection carburettor cars was America’s General Motors in 1991. That seems like an awfully long time ago now!

Nick Reid is head of automotive technology for Green Flag and a fellow of the Institute of the Motor Industry

12 comments on “Expert advice: what the ECU does and why it’s so vital for our car

  1. Geoff Arkell 12/10/2019 9:57 AM

    Many thanks for your articile on theECU Geoff Arkell

  2. Brian Sullivan 12/10/2019 7:55 PM

    I still prefer the old system , it was so easy to know what’s gone wrong and easy to fix. But these days you get so much wrong information like telling me the door is not shut when you know it is shut or a little light comes on for no apparent reason. To much rubbish.

  3. John Tucker 13/10/2019 9:33 AM

    VG thank you

  4. George Proctor 14/10/2019 9:16 AM

    I have a VW Tiguan. sometimes a caption comes on (Running on 2 cylinders?) How does this work? what are the other 2 cylinders doing. Regards GP.

  5. Brian Kidd 14/10/2019 11:17 AM

    Very interesting information in this item.With the growth of more complex technology the ordinary driver is being more and “distanced ” from his /her car.

  6. Keith 14/10/2019 4:25 PM

    Modern car electronics are very clever but a nightmare when they go wrong. I was told that almost all car faults these days are electronic not mechanical. I had to replace my ECU, a Vauxhall Corsa, the ECU was sited in the wiper chamber, the most ludicrous place, very prone to flooding which is what wrecked my ECU. Vauxhall (HQ), denied it was a problem and had never heard of it before, the specialist, however, said they were well aware as they had also told them the Corsa of that year (2008) and earlier had a big problem. Strangely, Vauxhall re-sited the ECU on all later models, they were not interested in helping out with the cost of replacement, (around £900). There’s a lot to be said for buying an older much simpler car.

  7. Brian 14/10/2019 8:32 PM

    Totally agree with Keith, another rip off by the manufacturers like keyless entry, was told by my manufacturer to buy a crook lock to protect my car from cyber thieves DOH back to the 60’s.

  8. Bruce Dibben 15/10/2019 9:03 AM

    Something not mentioned by the expert is ecu plug connections. Verdigris, damp and even bad earhting can play havoc and given a gentle but thorough clean of these connections can be all that a garage might do and charge you lots.

  9. JOHN 21/10/2019 8:38 AM

    Re your comment on the mass of sensors in today’s cars. Like your PC at home the computer is only as good as the operator and, in the case of the car ECU, it is the many sensors that all must be in perfect working order. I had an ECU replaced only to find that the problem was that the engine TCD sensor was defective. Fortunately, I insisted on retaining the original ECU so I got my money back. Like Keith, the simpler the car the better!!

  10. G. PostleEngine 22/10/2019 7:24 PM

    Engine Fault light , comes, on, a Citroen C3.
    Fault not recognised. Can be taken off, but reappears.
    Can the sensor, be effected by speed bumps.??/?

  11. Walker E. Weise 12/10/2022 1:43 PM

    The ECU, or engine control unit, is a computer that reads sensors throughout the car and tells the engine how to run. It makes sure the air and fuel mixture is correct, and that the engine is running at peak efficiency. It’s basically the brains of the car. The ECU is vital to our car because it controls so many important functions.

  12. Ivan Sanders 14/01/2023 9:17 PM

    I’ve driven and ridden numerous vehicles of all ages since I took to the roads in the 1960s. And kept performance and repair records for all of those vehicles. Although I am only one voice, and so of no conclusive weight, I will state my own findings without apology as we can only speak of our own experiences with authority and truth:

    As ECU’s become both more complex, and more numerous, we suffer more and more ‘warnings,’ unnecessary trouble. and extortionate expenses. In more cases than not any revealed fault is no more than a not needed (vis a vis sensible design) failed computer part.

    I recall one five year old vehicle I owned five years ago which went into three of four matching main dealerships for diagnostics arising from a single fault. I was charged over £ 4,000. for parts being replaced, parts which turned out to be sound. The dealerships eventually gave up and recommernded that I scrap the vehicle. A regularly serviced vehicle having only covered 45k!

    I advertised the vehicle with full disclosure and was lucky to sell an £ 8,000. car for £ 900. Leastways I thought so at the time.

    My buyer telephoned me two or three days later stating that he had fully repaired. All that was needed was a new earthing screw. So it cost him nothing.

    Whilst that was an extreme incident, such has repeated itself more than once more to lesser financial degrees, each time evidencing that being charged £ 95. plus VAT / hour does not secure a fully trained mechanic, simply a fitter with a gizmo God.

    I am fortunate enough to be able to afford a new / modern car but would not consider anything post 2,000 as a latest manufacture date. The majority of new UK cars are not even undersealed with essential hot zinc coating. Until that is undertaken incurring build costs around computers is beyond insane.

    To state that such gizmos are essential is patently false.

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