How to read a check engine code (and what to do next)

Every car sold in the US since 1996, in Europe since 2001 (petrol) and 2004 (diesel), and most cars elsewhere since then, has an OBD-II diagnostic port. When the check engine light comes on, the engine computer has stored a diagnostic trouble code (DTC) that tells you which system it is unhappy with.

1. Find the port

The OBD-II port is a 16-pin socket under the dashboard on the driver side, usually near the steering column or above the pedals. Some cars hide it behind a small cover.

2. Plug in a scanner

With the ignition off, plug in your scan tool or a Bluetooth OBD-II adapter. Turn the key to ON (engine off, or running if your tool says so) and choose "Read codes".

3. Write down every code and the freeze frame

Write down all codes, including pending ones. Freeze frame data shows the engine speed, temperature and load when the code was set, which helps a lot with diagnosis.

4. Look up the code

Type the code into the AutoDiag search. You will see what it means, how serious it is, the most likely causes in order, and the steps a mechanic would follow to find the fault.

5. Is it safe to drive?

A flashing check engine light means a severe misfire: slow down and avoid driving. A red oil pressure or temperature warning means stop as soon as it is safe. A steady check engine light with no other symptoms is usually safe for a short drive to a repair shop.

6. Do not just clear the code

Clearing a code turns the light off but does not fix anything, and it erases the freeze frame data a mechanic would use. Fix the cause first, then clear it and confirm it does not come back.