Used-car warning lights and OBD-II diagnostics

A dashboard light or scan-tool code is a clue, not a diagnosis; combine it with symptoms, records and an independent inspection.

Adapted from a guide first written for Quebec readers: passages specific to Quebec were removed. Rules in your province may differ.

A dashboard warning light should not be ignored, but an OBD-II code does not identify a failed part by itself. The same code can have several causes, and clearing it may hide useful information without fixing the problem.

If a warning appears while driving

Follow the vehicle manual. A flashing warning, overheating, loss of braking or steering, smoke, fuel smell or severe noise can require stopping safely and arranging assistance. Do not keep driving just to reach a preferred repair shop if the vehicle may be unsafe.

Use a scan as a starting point

• Record the exact code and any freeze-frame information before clearing anything.
• Note when the light appeared, recent repairs, fuel fill-ups and symptoms.
• Ask a qualified technician to interpret the code with tests and inspection.

Before buying a used vehicle

Request maintenance and repair records, check for warning lights, and arrange an independent pre-purchase inspection. A clean dashboard or recently erased codes do not prove the car has no problem. Review consumer-protection information for dealer and private sales.

Sources and limits

A diagnostic trouble code is not a diagnosis; have safety-critical symptoms assessed by a qualified technician.

General information based on federal and national rules; provincial rules may differ. Confirm with the official body before acting.