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

The check engine light means the engine computer has found a fault and stored a code for it. The code tells you where to look, and how urgent it is.

Updated 16 October 2026

What the light means

The check engine light, also called the engine management light or MIL, comes on when the engine computer finds a fault, usually in something that affects emissions. It stores a fault code for it, such as P0420 or P0171.

The light alone doesn’t say how serious the fault is. Some codes can wait until the next service; others mean you should stop.

Steady or flashing

Steady

The engine computer has found a fault. Read the code to see how urgent it is.

Flashing

Misfires bad enough to damage the catalytic converter. Stop when it’s safe to do so.

Red warnings, such as oil pressure or engine temperature, are different from the check engine light and more urgent: stop when it’s safe to do so and switch the engine off.

How to read the code

You need an ELM327 OBD2 adapter and a browser that can talk to it: Chrome or Edge on Android or a computer, or the free Bluefy browser on iPhone and iPad.

1. Plug the adapter into the diagnostic socket under the dashboard, within reach of the driver’s seat.
2. Switch the ignition on. The engine can stay off.
3. Open Read My Car and choose “Connect Bluetooth adapter” or, on a computer, “USB adapter”.
4. Look at the fault codes, with a plain-language explanation and how urgent each one is.

• Which adapter to buy
• Reading codes on an iPhone
• Does my car have OBD2?

What the code tells you

The first letter is the system: P for the engine and gearbox (powertrain), C for chassis, B for body and U for communication between modules. Generic OBD2 reaches the engine and often the gearbox; ABS, airbag, body and comfort modules need the manufacturer’s own protocol and tools.

Codes starting with P0 or P2, such as P0420, mean the same on every car. Codes such as P1xxx are manufacturer-specific, and their meaning depends on the make and model.

Stored

Confirmed. They turn the light on.

Pending

Seen once. Stored if they happen again.

Permanent

Can’t be cleared. The car removes them after the repair.

• Common fault codes explained

Clearing the code

Clearing doesn’t repair anything. If the cause is still there, the light comes back. Clearing also resets the emissions self-tests that inspections reading OBD check, so repair the cause first.

• Clearing codes before an emissions test

Questions