How to read OBD2 fault codes

Every petrol car sold in the US since 1996, and in the EU since 2001 (diesels from 2004), has an OBD2 port. A simple code reader shows why the check engine light is on.

DifficultyEasy
Time5–10 min

Tools

  • OBD2 code reader, or a Bluetooth adapter with a phone app

Step by step

  1. Find the OBD2 port: a 16-pin socket under the dashboard on the driver's side, within reach of the steering wheel.
  2. With the ignition off, plug in the reader.
  3. Switch the ignition on (engine off or running, as the reader instructs).
  4. Read stored and pending codes. Write them down, along with freeze-frame data if available.
  5. Look up each code in our OBD2 database.
  6. Fix the cause before clearing codes. Clearing codes also resets readiness monitors, which some emissions tests check.

Safety: Clearing a code does not fix the fault; it will come back.

Symptoms this can fix

Rough idleEngine cranks but won't startTransmission slippingCheck engine light onCar stalls at idle or when stopping

Procedures differ between vehicles. Check your vehicle's service information for specifications, torque values and any special steps.

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