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The check engine light: steady, flashing and what to do

The orange engine shaped light is the OBD warning. It doesn’t say what’s wrong: it says the car has stored a fault that affects emissions. This is what’s worth knowing.

It comes on and goes off when you switch the ignition on

That’s normal: the car is checking that the light works. If it goes off when the engine starts, nothing is confirmed. On some cars it stays on a few seconds longer, and that’s fine too.

Steady

The car has confirmed an emissions related fault and stored it. It can be something as silly as a loose fuel cap or something serious. You can almost always keep driving normally until it’s looked at, unless the car feels wrong: it loses power, jerks, smells of burning or the temperature climbs.

Flashing

It’s the urgent OBD warning: misfires so bad that unburnt fuel reaches the catalytic converter and can overheat it until it’s ruined. Ease off, don’t accelerate hard and, if it keeps flashing, pull over as soon as you can. It’s usually a coil, a spark plug or an injector.

With the engine down on power

If as well as the light the car won’t go past a certain rev limit or won’t pull, the control unit has gone into limp mode to protect itself. You can get to a garage calmly without pushing it, but it’s not for covering long distances.

Why it goes off by itself

If the fault doesn’t show up again over three drive cycles in a row, the light goes off by itself. The fault doesn’t vanish on the spot: it stays stored as history and clears itself after about forty starts with the engine reaching operating temperature without it happening again. That’s why it’s worth reading it even if the light has already gone off.

The car also counts the distance and time driven with the light on, and in Spain the ITV checks it: see the ITV and OBD.

Other lights that aren’t this one

The battery, oil, temperature or ABS lights warn about other things; a red oil or temperature light means stop now. The particulate filter light says it needs to regenerate. And on many diesels, the glow plug light flashing with the engine running acts as the engine fault light.

How to find out what’s wrong

With an OBD2 scanner and Final OBD Free you read the faults of every control unit for free with a short description and clear the engine ones; with Pro you also get the full explanation and how the car was running when they were logged. Clearing turns the light off, but if the cause is still there, it comes back. It’s all in how to read and clear fault codes, and what each code means in the list of codes.