Engine management light: 10 reasons why check engine warning light could be on

Got a check engine management light showing on your dashboard and not sure whether it's serious? We list the top 10 possible causes

Chris Rosamond
Jun 30, 2021

It’s only a little warning light, but if the check engine management symbol on your instrument panel illuminates while you’re driving, you’d be foolish to ignore it. Seeing that little orange outline of an engine light up during a journey doesn’t usually mean your motor is in imminent danger of implosion, explosion or grinding to a halt, but it still incites a sense of panic in many drivers.

There’s always a chance that something is seriously amiss, of course, so it’s not worth taking a risk and ignoring an engine warning light even if the odds are stacked in your favour. If you don’t find out what’s going on at the earliest opportunity, there’s a chance that a relatively straightforward and cheap-to-fix problem could turn into something much more serious and expensive.

Cars these days have multiple warning lights, but they’re usually fairly straightforward to interpret, thanks to the use of standardised symbols such as an oil can or thermometer. The engine check light is more mysterious, as there are all manner of systems linked to engine performance that can trigger it.

We should all know what it looks like, however, as it’s one of the lights that glows briefly as you start the car - keep an eye out when you switch the car on if you don't know what it looks likes. On some cars the words ‘check engine’ will light up too. The warning light is supposed to go out within a second or two once the engine fires up, so if yours stays on or lights up while you’re driving, you need to know what to do.

An orange or amber light doesn’t usually mean you should stop immediately, but some cars have a red warning light for more serious issues. A red engine warning means it’s time to pull over and call your breakdown service, or at least check the meaning of the light in your vehicle handbook.

Initial actions can vary from stopping immediately at the side of the road to visiting to a local garage for a diagnosis as soon as you are able to.

What an engine warning light means

The engine management light (EML) to give it its proper name, doesn't indicate a specific fault. It could be alerting you to one or more issues, typically linked to exhaust emissions. Some are more serious than others.

The light is connected to the car's main computer, called the engine control unit (ECU). It takes its information from sensors dotted around the engine and exhaust system. If they report the wrong data, the ECU will trigger the engine warning light.

If the car is still driveable and making no obvious noises, reduce your speed and drive it to your local garage. Depending on the problem, the car may make that decision for you by going into so-called 'limp home mode', where its speed is reduced almost to a crawl. If you’re on a motorway, pull over safely, stop and call a breakdown rescue company. Read on for more information about the common causes and how to work out what the light means.

Check engine light: what the colours mean

Some cars display the engine warning light in different colours according to how serious the problem is. They may also flash. Check your vehicle handbook for the precise definitions for your specific car. The meanings below are typical:

Amber engine management light

This is an advisory warning, which indicates an engine issue that affects emissions. In most cases, you'll be able to continue your journey, but you should get your car examined as soon as possible. It's always best to check your handbook for advice specific to your car.

Flashing engine management light (amber)

A flashing check engine light is likely to indicate a misfire in the engine, so the car may judder and could sound rough. Continued driving could overheat and damage the expensive catalytic converter, which is part of the exhaust system. You should immediately drive to a garage, using the accelerator gently.

Red engine management light

Some cars have a red check engine light that illuminates in the case of a serious fault. If you see this, you should stop when it’s safe to do so, in order to prevent serious damage. Don't drive the car but call your breakdown recovery provider to have the car checked or recovered to a qualified garage that can investigate the issue.

Engine light meaning: how to work it out

When a check engine light illuminates, your car also generates error codes, which indicate the specific problem. Garages download these using code readers that plug in to a data port, fitted to every modern car.

These diagnostic readers can be bought by anyone and are often universal, so will work for any make of car. Particularly popular are Bluetooth readers, which plug in to the data port (called OBD-II) and communicate wirelessly with your phone, allowing the codes to be downloaded to your device. You can then use online manuals to work out what the codes mean. Our guide to the most common causes, below, explains what to do next.

In some cases, the codes can reveal a quick fix, but that's not always the case, according to Frank Massey, an expert in vehicle diagnostics whose company, Auto Inform, trains vehicle technicians in their use. "The fault code reflects the symptom but not its cause," he says. "It might mean ‘faulty sensor’ which might be true but the environment it’s operating in might actually be at fault or it may have a poor connection. That being so, you replace the sensor only for it to produce another error code. You need to know much more about vehicle diagnostics to use them successfully."

Universal code readers may miss extra details that are specific to manufacturers. For example, Volkswagen and Audi use additional codes that can only be read by specialist readers.

How to switch off an engine warning light

Some older cars include instructions on resetting the light in the vehicle handbook. However if the fault isn't fixed, then it will simply come back on again when the car is started. Some code readers can also reset the warning light although, again, you'll need to address the issue first. Garage technicians reset the light when the issue is resolved.

Can I sell a car with an engine management light on?

There are so many causes of an engine management light that you should get the problem diagnosed first, as it could be a simple fix that costs just a few pounds to fix. However, if it indicates a serious fault with any of the exhaust parts that clean up emissions - such as a catalytic converter or particulate filter - then the bill could run into thousands of pounds.

If you're looking to trade the car in, then any repair costs will be priced into the offer. If a potential buyer doesn't know the reason why an engine warning light is illuminated - and consequently, how much this will cost to fix - you can expect them to offer you a much lower price than if the light wasn't on. Buyers and dealers are particularly wary of engine warning lights as these now cause cars to fail MOTs until resolved.

10 common reasons for an engine management light

1. Mass airflow sensor

Risk of damage Low
What to do Drive at reduced speed to the nearest garage

The mass airflow sensor measures the quantity of oxygen coming into the engine. Too little and the engine will run badly. For example, it may idle roughly or not accelerate smoothly. The emissions are likely to be affected, too, although this may not be so apparent. A reduction in airflow to the engine could be caused by a blocked air filter, a component that should be changed every couple of years.

2. Faulty oxygen sensor

Risk of damage Low
What to do Drive at reduced speed to the nearest garage

While the airflow sensor measures the quantity of air coming into the engine, the oxygen sensors (there are at least two on modern cars) measure the quantity leaving it via the exhaust system. The sensors report any abnormal deviation to the engine control unit which triggers the warning light.

3. Faulty exhaust gas recirculation valve (EGR)

Risk of damage Low
What to do Drive at reduced speed to the nearest garage

This device is critical to the clean running of your engine. It diverts up to 15% of the exhaust gas back into the engine where it’s burned. It can become bunged up with carbon deposits and stick open or shut. You may feel this as rough idling and misfiring, or difficult starting. A sensor will detect the issue and alert the ECU, which could activate a warning light.

4. Leaky vacuum hose

Risk of damage Low
What to do Drive at reduced speed to the nearest garage

Vacuum hoses channel air and emissions around the engine ensuring it runs efficiently. They’re exposed to extremes of temperature and being rubber, can deteriorate and become cracked, upsetting the delicate balances of gases. This variation is detected by sensors that report back to the ECU, which triggers the engine warning light.

5. Faulty ignition system

Risk of damage Medium
What to do Drive at reduced speed to the nearest garage

This is the system that is responsible for making sure the fuel and air are ignited correctly in the engine. In a petrol engine, there may be a problem with the system that decides when to make the spark that ignites the fuel or with the component (called the coil) that generates the electrical power that makes it. That being so, you’ll feel the engine misfire, a risky situation since not only does the car’s performance suffer but systems such as the catalytic converter could be damaged.

6. Blocked fuel injectors

Risk of damage Medium
What to do Drive at reduced speed to the nearest garage

The fuel injectors do exactly that: squirt fuel into the engine. There’s one per cylinder and the precise point at which they inject the fuel is determined by the ECU. The problem is they can become clogged by particles that have made it through the fuel filter, for instance, and be thrown out of kilter by problems elsewhere in the system or simply fail. This upsets the whole running of the engine which you’ll feel as misfiring or a sharp decrease in performance - and see as the warning light.

7. Blocked fuel pump

Risk of damage Medium
What to do Drive at reduced speed to the nearest garage

The fuel pump delivers fuel to the injectors under very high pressure. Any problems with it will upset that delivery of fuel and cause the engine to run badly. The engine warning light will come on and the engine itself may even go to limp-home mode, limiting the amount of power available to protect the engine.

8. Contaminated catalytic converter

Risk of damage High
What to do Drive at reduced speed to the nearest garage

The catalytic converter is responsible for cleaning up the car’s exhaust emissions but can itself be damaged or even destroyed by, for example, unburned fuel igniting in it. You might not notice any change in the car’s performance but because it’s an expensive piece of equipment, it's worth reducing your speed and getting it fixed as soon as possible.

9. Sooty particulate filter

Risk of damage High
What to do Drive at reduced speed to the nearest garage

Usually associated with diesel engines but now being introduced into petrol cars, the particulate filter captures the soot created when fuel and air are burned in the engine. If the filter gets clogged, the ECU will trigger the particulate filter warning light and you may be instructed to increase the engine speed to make the engine hotter and burn off the soot. If things get more serious, the engine warning light is likely to come on due to - among other things - the change to the engine’s oxygen supply. Get the problem resolved because a damaged filter is very expensive to replace.

10. Loose filler cap

Risk of damage Low
What to do Check or replace the fuel filler cap

As fuel is pumped out of your car’s tank, air must be allowed in to replace it or the resulting change in pressure upsets the flow of fuel. It gets into the tank via special vents that allow metered quantities of it to pass in. A loose fuel cap risks allowing in more air, which throws out the reading coming from the sensor that monitors the tank pressure. The ECU acts on this reading by triggering the engine warning light. It’s the simplest fault to cure but only through replacing the cap and eliminating it as a possible cause, will you know it was to blame.

 

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