If you’re driving and the check engine light comes on suddenly, it can be a bit alarming, especially if you’re an inexperienced driver or don’t know much about cars. The check engine light indicates there’s a minor or serious problem with your car’s engine or emissions system, and can come on for a wide range of reasons. So, what can cause the engine light to turn on?
The most common reason for your check engine light to come on (and the easiest problem to fix) is a loose fuel cap. This should be the first thing you check. If this isn’t the problem, it could be an issue with your spark plugs, oxygen sensor, catalytic converter or something else - it’s best to take your car in to be looked at by a reputable garage as soon as possible.
Keep reading for a detailed breakdown of what a check engine light means and the top ten most common causes, as well as what you should do about an illuminated check engine light.
The engine light, also known as the Malfunction Indicator Lamp (MIL), will usually illuminate for a few seconds when you first start your car and then switch off. If the MIL comes on at a different time or stays on longer than the first few seconds, this is a sign that there is a problem with your car’s engine or emissions system. It’s a signal from the engine control unit (ECU) that something is not working properly and that you should book your car in at a reputable local garage to be looked at as soon as possible.
The check engine light is a small indicator located on your dashboard with the silhouette of an engine. It usually illuminates when something is wrong with your engine or gearbox. The light can be a solid colour (typically yellow, orange or amber) or a blinking orange/red depending on the severity of the issue. This could change for each manufacturer, so always best to check your handbook.
A solid coloured check engine light indicates that there is a minor problem with your engine. Although you should get your car looked at as soon as possible, there is no cause for alarm, and you can continue with your journey as normal. However, a blinking check engine light indicates a much more serious problem, and you should pull over as soon as it’s safe to do so. You should then arrange for your vehicle to be recovered by your breakdown service or by a local garage. Continuing to drive with a blinking check engine light can be very dangerous, as it indicates a serious fault.
It’s worth bearing in mind that some models of car might show a solid red check engine light to indicate a serious issue, rather than a blinking one. If your check engine light is a solid red colour, compared to its usual yellow or orange, you should treat it the same as a blinking check engine light.
There are a wide range of reasons why your check engine light might be on, some of which are very serious. Here’s an overview of the most common causes for your check engine light to be illuminated.
A loose or improperly sealed fuel cap can cause your check engine light to illuminate. This is because a loose fuel cap can lead to fuel vapour leaks, which can trigger an error in the emissions system. If your check engine light comes on, it’s always worth checking that your fuel cap is properly secure before you start looking at other possible causes.
Your car’s oxygen sensor is responsible for measuring the amount of oxygen in the car’s exhaust system. The oxygen sensor’s data is then sent to your car’s computer (ECU), which adjusts the fuel-air mixture in the combustion chamber. If this sensor malfunctions or fails, it can send an error code to the engine control module (ECU), which will trigger the check engine light. You might also notice that the engine is running poorly and your car struggles to accelerate.
This catalytic converter is responsible for reducing harmful emissions. If it fails or becomes clogged or damaged, it can trigger a fault code which then activates the check engine light. This can be caused by carbon deposits, which build up inside the catalytic converter over time. A sulfurous, or rotten egg, smell coming from your exhaust is a telltale sign that there is a problem with your car’s catalytic converter.
When spark plugs fail or become faulty, they may not properly ignite the air-fuel mixture in your car engine's cylinders, causing a misfire. Your car’s computer can detect misfires and will trigger the check engine light to alert you to a potential problem. Other signs that there is an issue with spark plugs include difficulty starting the engine and reduced fuel efficiency.
Spark plugs need to be replaced regularly (typically between 30,000 and 150,000 miles, depending on the type), so if you have a high-mileage car and you have not had the spark plugs replaced recently, this could be the cause of your illuminated check engine light.
If your check engine light is due to a faulty spark plug or any other issue, why not book your car in at a reputable independent garage near you using the Approved Garages network?
A low or faulty battery means low voltage to your car’s computer, which can cause error codes and activation of the check engine light. Loose or corroded battery terminals can also cause similar issues. Other symptoms of a dying battery include a clicking noise when starting your car, dim headlights and difficulty turning over.
A worn or slipping clutch can put stress on other gearbox components, leading to transmission problems that can illuminate the check engine light. Other symptoms of a clutch problem include difficulty shifting gears, vibrations coming from the gearbox and grinding noises when you change gear.
Your car’s mass airflow (MAF) sensor measures the amount of air entering the engine and sends this data to the Engine Control Unit (ECU) to ensure the proper air-fuel mixtures. If the sensor fails, malfunctions or becomes damaged, it can lead to incorrect air-fuel mixtures, which can result in an error code. This then activates the check engine light. You might also notice that your car stalls more frequently, and it jerks when accelerating.
An improperly installed or malfunctioning aftermarket car accessories can interfere with your car’s electrical system, including the Engine Control Unit (ECU), leading to errors which trigger the check engine light. This interference can cause false readings or signal loss from a range of sensors in your vehicle.
A vacuum leak means extra air is introduced into the engine, affecting the air-fuel mixture and triggering the engine control unit to illuminate the check engine light. This improper air-fuel mixture can also lead to other issues, such as the engine stalling or sputtering.
The exhaust gas recirculation (EGR) valve is responsible for recirculating some of your car’s exhaust gases back into the engine cylinders. If this valve fails or becomes faulty, it can lead to an improper air-fuel mixture, which can trigger the engine control unit to switch on the check engine light.
Other symptoms of this include reduced engine performance, poor fuel economy and a decrease in acceleration.
If you notice the check engine light has turned on while you're driving, the first step is to check whether it’s a solid amber, yellow or orange light (indicating a minor problem) or a flashing or red coloured light (indicating a serious issue with your engine). If the light is solid, it’s generally safe to continue driving to your destination, but you should book your car in for a diagnostic check at a local garage as soon as possible. However, if you notice any other issues with your car, this could indicate a more serious problem.
If the light is flashing (or red-coloured in some car models), this indicates that there is a serious issue with your engine or gearbox. You should pull over as soon as it’s safe to do so and contact roadside assistance or a local garage to arrange recovery of your car. It can be highly unsafe to drive with a flashing check engine light, especially if you have a long way to travel or are travelling at high speeds.
Before you get a diagnostic check, it’s always worth checking that your fuel cap is on correctly, as this can trigger the check engine light.
Yes, provided the check engine light is a solid yellow, orange or amber colour, it’s generally safe to continue driving. However, you should still book your car in for a diagnostic check at a local garage as soon as possible. The Approved Garages network makes it easy to find a reputable independent to check your car over and diagnose the problem that’s illuminating your check engine light.
If your check engine light is flashing or is a solid red colour, we recommend pulling over when it’s safe to do so and seeking roadside assistance or getting a local garage to tow your car. You should then get a professional mechanic to complete a full diagnostic check to confirm the fault and repair the issue. A flashing check engine light indicates a serious engine or gearbox fault, and continuing to drive could be highly unsafe.
The check engine or engine management light (EML) should reset itself after the issue has been resolved. You should not attempt to reset the check engine light by yourself, as this will mean that the issue has not been fixed, and your car could still be unsafe to drive.
You can book your car in a local reputable garage in just a couple of steps using the Approved Garages network.
With Approved Garages, it’s simple to find a professional local garage to diagnose and repair your vehicle. All you need to do is enter your registration number and location to book your diagnostics check in just a few clicks!
All the garages in our network have experience diagnosing and repairing a wide range of engine and gearbox faults that can cause the check engine light to turn on, from simple spark plug replacements to problems with your catalytic converter. All repairs are carried out using Original Equipment Matching Quality parts, so you can be sure you’ll receive a high-quality and competitively-priced service.
Get started by searching for trusted local garages near you, and book your diagnostic check and repair today.
A check engine light will only resolve itself after the issue has been fixed. If your car has been repaired and the check engine light remains on, it may take a few cycles of turning your car off and on again for the light to turn off automatically. If the light remains on for longer than this, you should take your car back to the mechanic, as there might be another problem or the underlying cause of the illuminated light might not have been fixed.
A check engine light can come on for a range of reasons, and it doesn’t always mean that there is a serious problem with your engine or emissions system. It can come on for minor issues such as faulty sensors, loose connections or even a loose fuel cap.
It’s a good idea to check that your fuel cap is properly secured before you take your car to be looked at. However, if this isn’t the cause of the illuminated light, it’s still best to take your car for a diagnostic check so that the root of the problem can be identified. It might be a minor issue, like a faulty sensor, but it could also be something much more serious.
An orange (or amber) check engine light generally indicates a less serious issue with your engine or emissions system. It should be safe to continue driving, but you should take your car to a trusted local garage as soon as you can.
Yes, generally having a check engine light illuminated will cause your car to fail its MOT. It will be considered a major defect and lead to a failed test. If your car’s check engine light is on, it’s best to have it diagnosed and repaired before your MOT to avoid the hassle of a certain fail and a retest. You can book your car in for a diagnostic check quickly and easily at a garage near you, using the Approved Garages network.
While it’s not illegal to drive with the check engine light on, it is strongly recommended to get it diagnosed by a reputable mechanic as soon as possible. If you have a flashing or red coloured check engine light (indicating a serious problem) and visible issues with your car, you could be pulled over by the police and fined for driving your car in a dangerous condition and receive three penalty points on your license.
Similar to an orange check amber light, a yellow check engine light indicates there is a non-critical issue with your car’s engine or emissions system.
Yes, a loose or improperly closed fuel cap can cause the check engine light to come on. The fuel cap is part of the EVAP (Evaporative Emissions) system, which helps prevent fuel vapours from escaping. A loose cap can lead to vapours escaping, which the vehicle’s computer may detect as a problem and trigger the check engine light.
