My Check Engine Light Is On — How Much Will the Diagnostic Cost?

A professional diagnostic typically costs $100–$180 in Canada, and many shops credit that fee toward the repair if you go ahead with them. That covers a technician's time with a proper scanner — not just reading the code, but actually testing to find the root cause.

Parts stores sometimes read codes for free, but a code like “P0420” only points to a system, not a specific part. Replacing parts based on a free code read is how $50 sensors turn into $600 guessing games.

Do not ignore a flashing check engine light. Flashing means an active misfire that can destroy the catalytic converter, turning a small repair into a very expensive one. A steady light is less urgent, but book a diagnostic this week, not next month.

Common causes range from a loose gas cap — a free fix — to oxygen sensors, ignition coils, or catalytic converters, spanning roughly $200 to $1,500 or more depending on the part and the car.

Once you have the diagnosis, the repair itself still deserves a price check. Look up the fair range for whatever they found before you authorize the work — the diagnostic tells you what is wrong, and the price check tells you whether the quote is fair.