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

A check engine light that comes on (the yellow or orange engine symbol, also called the engine management light) means the car's engine control unit has detected a fault in the engine or the emissions system and stored it as a fault code. If the light is steady, you can keep driving carefully, but get it checked soon. If it is flashing, slow down and stop as soon as it is safe: it usually points to misfires that can damage the catalytic converter.

What is the check engine light?

It is the warning light known in technical jargon as the MIL (malfunction indicator lamp). It is shaped like an engine, sometimes with the word "CHECK", and lights up in yellow or orange.

When you turn the ignition on, it lights up for a couple of seconds and goes out: that is the check that the bulb works. What matters is whether it stays on with the engine running.

Behind that light is the on board diagnostics system, OBD. Since 2001 for petrol cars and 2004 for diesels, European rules require cars to monitor everything that affects emissions: the air and fuel mixture, ignition, oxygen sensors, the catalytic converter. When something goes outside normal limits, the engine control unit turns the light on and stores a code that explains what has happened.

Check engine light steady or flashing: what should I do?

Steady

The fault has been detected, but it is not immediate. It can be something as simple as a fuel cap that was not closed properly, or as serious as a sensor giving false readings that make the engine burn more fuel. You can keep driving carefully, but do not leave it for weeks: read the code or book the car into a workshop.

Pay attention to how the car drives as well. If you also notice jerking, loss of power, smoke or strange noises, treat it as more urgent.

Flashing

This is more serious. It normally means misfires: one or more cylinders are not burning the fuel properly, and that unburnt fuel reaches the catalytic converter, which can overheat and be ruined.

What do the dashboard warning light colours mean?

The check engine light is yellow, so it is not an emergency unless it is flashing. But if a red light comes on at the same time, the red one takes priority. The battery light, for example, warns of a charging problem; we explain it in our article on the car battery in winter.

What is an OBD fault code (DTC)?

When the light comes on, the engine control unit stores a fault code, or DTC (diagnostic trouble code). It always has the same structure: one letter and four digits.

• The letter gives the system: P for engine and transmission (powertrain), B for body, C for chassis and U for communication between control units. With the check engine light, it is almost always a P.
• The first digit says whether it is a generic code, the same for every make (0), or one specific to the manufacturer (1).
• The second digit gives the subsystem: for example, 1 is air and fuel metering, 3 is ignition, 4 is auxiliary emissions systems.
• The last two digits pin down the fault.

Three common codes, explained

• P0171: system too lean (bank 1). The engine is getting more air or less fuel than it should. It can come from an air leak, a dirty mass air flow sensor, an injector or the fuel pressure. The code tells you what is happening, not always why.
• P0300: random misfires. Several cylinders misfire with no fixed pattern. If the misfire is in one specific cylinder, the code ends in its number (P0301 for the first, P0302 for the second...). Spark plugs, coils or injectors are the usual suspects. This is the classic code behind a flashing light.
• P0420: catalyst efficiency below threshold (bank 1). The catalytic converter is not cleaning the exhaust gases as it should. Sometimes it is the converter itself; other times, an oxygen sensor reading wrongly.

Can I read the fault code myself?

Yes. Every car with OBD has a 16 pin connector, usually under the steering wheel. With an OBD reader you can read it in a couple of minutes:

1. Find the OBD connector (under the steering wheel or near the fuse box; if you cannot see it, check the handbook).
2. With the car switched off, plug in the reader.
3. Turn the ignition on or start the engine, as the reader instructs.
4. Read the stored codes and write them down.
5. Look up what each one means and whether it is urgent.

There are readers with their own screen and others that connect to your phone over Bluetooth. The trouble with many of them is that they give you the code and a technical phrase, and you are none the wiser.

The check engine light and the ITV in Spain

Driving into the ITV (Inspección Técnica de Vehículos, the Spanish equivalent of the British MOT) with the engine warning light on counts as a defect, so it is best to fix it before you go. In addition, the inspection stations' procedures manual, in the version in force since January 2026 (BOE-A-2025-20343), includes reading the OBD system as part of the emissions test for Euro 5 and Euro 6 cars: the inspector plugs their equipment into the same connector you use.

If the car fails the inspection, Royal Decree 920/2017 (Real Decreto 920/2017) gives you a maximum of two months to fix the defect and go back. If you are not sure when yours is due, see when your ITV inspection is due in Spain.

So it does not slip your mind

A warning light does not fix itself, and the worst thing that can happen is getting used to seeing it. If it comes on, send a photo of the dashboard to Jack, our WhatsApp assistant: he tells you what that light usually means and notes down the trip to the garage so it does not slip your mind. He does not replace the mechanic, who is the one who reads the code and diagnoses it.

The Vehistrack car tracker plugs into that same OBD connector. It logs the real mileage by itself and keeps an eye on the battery voltage, which is exactly what you need to follow a car maintenance schedule by mileage. How much data you get depends on the make, model and year of the car.

Frequently asked questions

Can I pass the ITV with the check engine light on?

You can turn up, but a lit engine warning light counts as a defect. The sensible thing is to read the code and fix the cause before you go.

Does the check engine light go off by itself?

Sometimes, yes. If the fault was a one off (a fuel cap left loose, for example) and it goes away, the engine control unit may switch the light off after several journeys. The code usually stays in the memory as a past fault.

Is it dangerous to drive with the check engine light on?

If it is steady and there are no symptoms, you can normally get to a workshop if you drive carefully. If it is flashing, or there is loss of power, jerking or smoke, stop as soon as it is safe.

What is the difference between the check engine light and a serious warning light?

The engine light is yellow or orange and warns of a fault in the engine or emissions. Red lights (oil, temperature, brakes) signal a problem that requires you to stop.

Which OBD reader do I need?

Any reader compatible with the OBD standard can read generic codes. For manufacturer specific codes, you sometimes need a more advanced reader or a workshop.