Will a vehicle pass an inspection with the check engine light on?
A pre-purchase vehicle inspection is a great way to make sure you’re buying your next vehicle with confidence. As well as checking the mechanical and cosmetic condition of the vehicle, electrics will also be reported on – so, what happens if the engine warning light is on?
Here, we’ll look at what impact an engine warning light will have on a vehicle inspection, as well as what it could mean for the health of the car you’re looking to purchase.
What is a ‘check engine’ light?
If you’re a driver, you’ll see your engine management light come on every time you start your car. It’s an amber coloured light that looks roughly like the rectangular outline of an engine that’s found behind the steering wheel, in your instrument cluster. Don’t worry though, like your oil or airbag warning light, it generally only lights up for a couple of seconds when you switch the electrics in your car on – then goes off.
That said, sometimes your engine warning light will stay on or may come on when you’re driving – sometimes to alert you to a serious problem with the car, and sometimes just because of a minor fault.
Since your engine warning light can come on for a variety of reasons, having one showing on a car that you’re thinking about buying is a cause for concern.
What does the engine warning light mean?
There’s no single reason that your engine management light will come on. In actual fact, the light is connected to your car’s ‘Engine Control Unit’ – or ECU. The ECU is the ‘brain’ of the car, a computer that manages everything from fuel to braking systems. Since the ECU controls so many different things – there’s a long list of potential problems that an engine warning light could be telling you.
This long list of potential problems can only really be explored in more detail with specialist computer software. Fortunately, dealers, garages, and our inspectors carry this diagnostic equipment. When a laptop with the appropriate software is connected to your car, it will read ‘fault codes’ that are registered on the ECU. Each of these fault codes indicates a different problem – some are easily fixed – but others can represent really big problems.
What are some common reasons for the engine management light staying on?
There are often hundreds of potential problems that your engine warning light could be trying to alert you to – but there are a handful of common issues that come up for many manufacturers. They include:
Mass airflow sensor problems
The mass airflow (MAF) sensor analyses a range of conditions and decides how much fuel and air your engine needs run as efficiently as possible. If there’s a problem with the sensor, your fuel/air balance can be incorrect, causing the car to run poorly – or even switch into ‘limp mode’ meaning the car will not move at more than a few miles per hour.
A MAF problem will usually show up as an engine warning light on your dash.
Ignition system problems
Although the system is slightly different between petrol and diesel vehicles, at some point, engines need either heat or a spark to ignite the fuel/air mix and get the engine running. When there’s a problem with this system, the car will either struggle to start or might ‘misfire’ – meaning power is lacking or intermittently drops off.
Problems with this ignition system generally show up as an amber engine warning light.
A fuel tank issue
You might not realise it, but your fuel tank ‘breathes’. As fuel leaves and is used by the engine, it’s replaced with air – so that a constant pressure is maintained and the sensors that decide how much fuel is drawn into the engine can keep the system running properly.
If the pressure in the tank is slightly wrong, the ECU will consider there to be a fuel problem, and the engine warning light will show up again.
DPF problems
On diesel cars, a DPF (diesel particulate filter) removes dangerous particles from exhaust gases. After a while, this filter can become clogged up with a sooty residue – and the engine will deal with this by periodically burning a little extra fuel to increase the pressure on the DPF and burn the soot off.
The problem is, this heat build-up requires the car to run for ten or more minutes – and that simply might not happen if you rarely do more than a few miles. As such, this DPF can become more and more blocked up – meaning the engine struggles to vent exhaust gases as it should. The result? The engine warning light will often come on.
Emissions system faults
Throughout your exhaust system, there are sensors that monitor the amount of oxygen that’s released after combustion has taken place in the engine. Too much oxygen would indicate to the ECU that the air/fuel mix is incorrect.
An incorrect fuel mixture would be enough to trigger the engine warning light – but, as is more often the case, so would a broken oxygen sensor that’s providing an incorrect reading. Either way – the engine management light will come on.
What will an inspection report say if the engine management light is on?
Since a pre-purchase inspection isn’t designed to ‘pass’ or ‘fail’ a vehicle, it will simply show if there are any engine management issues with the car – and, if so, what they are.
In the Road Test section of the report, there is a line that indicates whether the Warning Message/Light is showing. If it isn’t, this will be marked as ‘OK’ (green). If the light is showing, it will be marked as either ‘Warning’ (amber) or ‘Danger’ (red) – depending on the reason it’s come on.
In the Diagnostics section of the report, any fault codes registered on the ECU will show – and, if the engine warning light is on, you can expect to see why here. The code will be shown, as well as which system the code relates to, and a description of the problem it indicates.
Again, none of this information will ‘fail’ a car as such – but it is likely to be cause for concern, and gives you all the information you need to decide whether it’s a problem you’re willing to take on with your new car – or whether it’s time to look elsewhere.
