Can I drive with the check engine light on?

If the light stays on steadily and the car drives normally, you can usually keep driving at a moderate pace, but have the code read soon. If the light flashes, a red warning light comes on or the car behaves differently from usual, stop when it is safe.

What does a flashing check engine light mean?

A flashing light usually means severe misfires. Unburnt fuel can destroy the catalytic converter, so stop when it is safe and do not drive on.

Will my car pass its MOT with the engine management light on?

In the UK, an engine management light that shows a fault is a major defect, so the car fails its MOT. This applies to most petrol cars first used from 1 July 2003 and diesel cars first used from 1 July 2008. Elsewhere in Europe it depends on your country’s inspection rules; the EU roadworthiness directive lists an OBD read-out showing a significant malfunction as a reason for failure on petrol cars, and a fault that affects emissions can also fail the exhaust test. It is safer to fix the fault before the test. If the car fails, it has to be repaired and tested again.

Can I clear the code and turn off the light myself?

You can clear the code with an OBD reader, but the cause remains and the light will come back on. Clearing it also wipes data the mechanic would need to find the cause. So write the code down or show it to a mechanic before clearing it.

Check Engine Light On or Flashing: What It Means

What the check engine light means

On most cars the light comes on briefly as a self-test when you switch on the ignition and goes out once the engine starts. If it stays on, the engine control module has detected a fault and stored it as a code, such as P0420. The light itself does not tell you what is wrong; only the code does. Causes range from a loose fuel cap or a faulty sensor to misfires or a worn catalytic converter.

Steady or flashing light

It matters whether the light is steady or flashing.

Steady: the engine control module has detected a fault. You do not need to stop straight away, but have it checked as soon as you can and watch for any change in how the car drives.
Flashing: this usually means severe misfires. Unburnt fuel burns inside the catalytic converter and can destroy it. Do not accelerate, stop when it is safe and do not drive on.

What to do when the light comes on

Check how the car is behaving and act accordingly.

The light is flashing: stop when it is safe and call roadside assistance or a garage.
A red warning light for oil pressure, temperature, brakes or battery charging is on as well: stop and switch off the engine.
The car drives differently from usual (less power, shaking, a smell, smoke): drive gently and only as far as a garage, or have the car recovered.
The light is on but the car drives normally: drive moderately and have the code read soon.
The light came on shortly after filling up: check that the fuel cap is closed properly; this is one possible cause.

How to read the fault code

Petrol cars from 2001 and diesel cars from 2004 have a standard OBD port. There are several ways to read the code.

A garage reads the codes with a diagnostic tool that can read all of the car’s control modules, not just the engine.
A basic OBD reader shows the engine control module codes, but not the detailed codes from the gearbox or other systems.
Once you know the code, enter it in Evoca’s free fault code checker, or take a photo of the garage printout, and get an explanation in English.

Common codes that turn on the check engine light

These codes often appear when the check engine light is on. Click a code for an explanation.

More codes explained

Evoca fault code diagnostics is an informational aid and does not replace a professional inspection of your vehicle. Only an inspection of the vehicle at a workshop can confirm the cause of a fault.

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Once a workshop or a scan tool has read the codes, enter them in the form. You get a free report by email.

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Check engine light on or flashing: what it means and what to do