Check engine light: Everything you need to know

If your check engine light (CEL) comes on, this indicates there is an issue with your car that requires investigation.

Unfortunately, this particular dashboard warning light doesn’t tell you exactly what that might be. You could be dealing with a loose fuel cap, an overheating engine, or another minor-to-major engine-related problem.

Whilst the specific nature of the problem may initially be unclear, it’s important to be aware that ignoring a CEL will compromise the performance and safety of your car. If you’re currently experiencing a minor issue, it’s only likely to get worse over time. However, leaving a more critical fault unchecked could have disastrous consequences.

In this guide, we’ll help you identify the check engine light, explain what each of its variations means, and list its most common triggers. We’ll also cover the action you should take when the light appears - and explain the process mechanics follow when diagnosing and resolving CEL issues.

Get a free valuation

What is the check engine light?

The CEL, sometimes known as the ‘engine management light’, is your car’s way of telling you that it’s detected an engine-related problem.

In some cars, this warning is depicted as an engine-shaped icon, whilst in others, text saying ‘Check engine’, ‘Service engine’, or a similar phrase will appear. Despite these variations, the CEL will always appear in two colours: orange (sometimes amber or yellow) - and red.

If the CEL illuminates, it’s important to be aware that action is required. The specific action that you need to take (and how quickly) will depend on the colour of the light and how it is behaving. We’ll explain this in more detail in the section below.

Different types of check engine lights

Steady yellow/orange/amber light

If your CEL is showing a steady, non-blinking yellow, orange, or amber light, the issue you’re dealing with is probably a minor one, such as a loose fuel cap, a worn-out oxygen sensor, or a fault with the mass airflow sensor. It’s safe to keep driving in this scenario, but you should arrange to have your car checked over by a mechanic promptly.
Flashing yellow/orange/amber light

A flashing yellow, orange, or amber light indicates there is a more serious problem (although not quite severe enough to justify a red light). Potential causes include faulty spark plugs, a misfiring engine, or fuel system problems.

These are all likely to cause damage to your car if you continue driving, potentially leaving you with a hefty repair bill. The best advice is to pull over as soon as it’s safe, then call your mechanic.
Red light

If your CEL is red when it comes on, and stays lit, this means your car has a severe issue (e.g. an overheating engine or dangerously low oil pressure).

Pull over as soon as it is safe to do so, turn off your engine, and call for roadside assistance.

Common causes for the check engine light

Loose fuel cap

If the fuel cap is loose, the fuel will start to evaporate, and air will get into the system. This change in the carefully calibrated air-fuel mixture will reduce your car’s fuel efficiency and increase its emissions.
Oxygen sensor failure

A faulty or worn-out O2 sensor will send inaccurate readings to the engine control unit (ECU) about how much unburned oxygen is in the exhaust system, leading to the air-fuel mixture being adjusted incorrectly. This, in turn, will lead to reduced fuel efficiency, increased emissions, and potential damage to the catalytic converter.
Faulty catalytic converter

If your catalytic converter is broken or blocked, it can’t convert the toxic gases as it should. A faulty catalytic converter will increase emissions, reduce fuel efficiency and engine performance - and potentially damage other parts of the exhaust system.
Ignition system issues

The ignition system consists of multiple components, including spark plugs, ignition coils, and an ignition switch.

Each of these parts were designed to help the engine start up and run smoothly. If any of them stop working properly, the engine is likely to misfire, which could damage the catalytic converter. Your car’s fuel efficiency and engine performance may also be affected.
Dead battery

If the battery is dead, your engine won’t start. If it’s recently been changed or disconnected, your electrical system may need to be reset. Either of these scenarios can trigger the CEL.
Mass airflow sensor problems

Any issues with the mass airflow sensors will lead to an incorrect air-fuel mixture, which, in turn, will cause problems such as stalling, rough idling, poor acceleration, and reduced fuel efficiency.
Blocked diesel particulate filter (DPF)

The diesel particulate filter (DPF) helps to reduce emissions from diesel cars by trapping soot and other debris. If it gets blocked, this can result in increased fuel consumption, poor engine performance - and damage to the engine.

What to do when the check engine light comes on

The correct course of action when you see a CEL will depend on how it appears on your dashboard:

Steady orange/yellow/amber light

If everything else in the car sounds and feels normal (and no critical dashboard warning lights appear), it is safe to continue driving. However, when you reach your destination, you should check the fuel cap. If it’s loose, you can tighten it yourself. For any other issues, take your car to a garage as soon as possible.

If you’re hearing unusual noises, your car isn’t handling correctly, and/or you’re seeing unrelated but critical dashboard warning lights, pull over as soon as it is safe to do so. Turn off your engine, call your breakdown provider, and arrange for your car to be looked at by a mechanic. Although the issue could be minor, it’s best to err on the side of caution. You don’t want to risk damaging your car.

Flashing orange/yellow/amber light

A flashing orange, yellow, or amber CEL could indicate a serious fault. To avoid damaging your car, you should pull over as soon as it is safe to do so - and arrange for a mechanic to investigate the issue.

Red light

A red CEL indicates a potentially dangerous issue such as an overheating engine, or low oil pressure. When this light appears, pull over immediately, turn off the engine, and call your breakdown provider.

Act quickly: Don’t ignore the CEL

Whatever type of CEL you’re dealing with, it’s important to take your car to a professional mechanic for a diagnostic test. They have the tools and expertise to pinpoint the main issue, along with any underlying concerns.

From here, the mechanic can carry out all the necessary repairs or replacements. If you ignore the CEL, or try to diagnose and fix the problems yourself, you risk damaging your car - and jeopardising your safety.

Diagnosing and fixing check engine light issues

When diagnosing CEL issues, mechanics will often use an OBD-II reader.

This device plugs into your car’s on-board diagnostic system (OBD-II) and provides information in the form of diagnostic trouble codes (DTCs), which the mechanic can then translate. This gives them a starting point - and may lead to further testing corresponding to the various DTCs.

Possible fixes for the common issues covered earlier include:

Tightening a loose fuel cap.
Replacing a faulty O2 sensor.
Replacing or repairing a faulty catalytic converter.
Replacing faulty spark plugs, ignition coils, or other parts of the ignition system.
Recharging or replacing a dead battery - or checking that it’s properly connected.
Cleaning or replacing the mass airflow sensor.
Cleaning or replacing the DPF.

It’s important to address all CEL issues as soon as possible to avoid severely damaging your car.

Can you reset the check engine light yourself?

Whenever you see the CEL, you should take your car to a mechanic. While there are simple things you can try to reset the check engine light, it’s better to get professional help to make sure any problems have been diagnosed and fixed.

There are two basic things you can do to potentially reset the CEL:

Check that the fuel cap is on properly - and if it’s loose, tighten it.
Turn the engine off, leave it for a couple of minutes, then restart it.

If either of these methods are successful in turning the CEL off, you should be safe to drive. However, if it comes on again, or you start to experience anything unusual as you’re driving, take your car to a garage as soon as you can.

Whether orange, yellow, amber, red, steady or flashing, your check engine light is not something you should ever ignore. If you do, you’ll risk your safety, whilst compromising your car’s efficiency and performance.

If the problems concern your exhaust system, you could also fail your MOT - or face a fine for illegal emissions.

Frequently Asked Questions

If the check engine light is orange, yellow, or amber (and not flashing), then it is safe to drive. You should arrange for a mechanic to check your car as soon as possible, though.

If the light is flashing or red, then you should stop driving immediately.

No, the check engine light won’t reset itself. You may be able to reset it yourself by restarting the car or tightening the fuel cap, but otherwise, you should get help from a professional mechanic.

Yes, if your oil level is low enough to affect the oil pressure, then that can trigger the check engine light. You might also have a low engine oil warning light on your dashboard.

If the light is steady orange, yellow, or amber, and there are no unusual sounds or vibrations, it’s safe to drive. However, you should visit a garage promptly.

If the light is flashing or red, you should stop driving immediately.

No, your car won’t pass its MOT with the check engine light on, as this is considered a major fault. You will also fail the emissions component of the test.

ABS (Anti-lock Braking System): Everything you need to know

Engine Temperature Warning Light: Everything you need to know

Battery Charge Warning Light: Everything you need to know

Power Steering Warning Light: Everything you need to know

Oil Warning Light: Everything you need to know

Transmission Warning Light: Everything you need to know

Low Fuel Warning Light: Everything you need to know

Tyre Pressure Warning Light: Everything you need to know

Traction Control Warning Light: Everything you need to know

Screen Wash Warning Light: Everything you need to know

Glow Plug Warning Light (Diesel): Everything you need to know

Diesel Particulate Filter Warning Light: Everything you need to know