When the Check Engine light comes on, your car is telling you a story. An OBD2 scanner ($25 at any parts store, or borrowed from a friend) reads the fault code that describes the problem. This guide covers the 15 most common codes we see in Quebec, in plain language.
How to read OBD2 codes
Every vehicle sold in North America since 1996 has an OBD2 port — a 16-pin connector under the dashboard, driver side. Plug in a scanner, start the car (or turn the key to ON) and it displays the stored codes.
An OBD2 code is always letter + 4 digits:
- P = Powertrain (engine, transmission) — 90% of codes
- B = Body (airbags, A/C, seats)
- C = Chassis (ABS, steering, electronic brakes)
- U = Network / module communication
The 15 most common codes in Quebec
P0300 — Random misfire (unknown cylinder)
One or more cylinders misfire. Likely causes, in order: worn spark plugs (replace first), then ignition coils, then clogged injector, then low compression. Cost: $50-400. See our spark plugs guide.
P0301 – P0308 — Misfire in a specific cylinder
Last digit = cylinder number (1-8). Same fix as P0300 but targeted. If only one cylinder: replace the coil and plug on that cylinder first.
P0171 — System too lean (bank 1)
Too much air or not enough fuel. Causes: dirty MAF sensor, vacuum leak, weak injector, clogged fuel filter. Cost: $80- 300. Start with MAF sensor cleaning ($15 spray cleaner).
P0172 — System too rich (bank 1)
Too much fuel, not enough air. Causes: MAP sensor, O2 sensor, leaking injector, clogged air filter. Start with a fresh air filter.
P0420 — Catalyst efficiency below threshold (bank 1)
Catalytic converter not working as well as it should. Causes: worn-out cat ($400-1500), bad O2 sensor ($80-200), chronic misfires that damaged the cat. Always check O2 sensors before replacing the cat — 10x cheaper and often the real cause.
P0430 — Same as P0420 but bank 2
For V6/V8 engines with two cylinder banks. Same diagnosis.
P0128 — Coolant temperature below thermostat regulating temperature
Engine not warming up enough. Almost always a thermostat stuck open. Part: $30-80, often DIY-able. Fix before winter — a cold engine burns more fuel and wears faster.
P0442 — Small EVAP leak
Evaporative emissions system that captures fuel vapor. Most common cause: loose or worn gas cap. Tighten the cap, clear the code, drive a few cycles. If it comes back, replace the cap ($15) before chasing deeper.
P0455 — Large EVAP leak
Same system, bigger leak. Missing cap, cracked EVAP hose, bad purge valve. Start with the cap.
P0456 — Very small EVAP leak
Similar to P0442, even more subtle leak. Same approach.
P0401 — Insufficient EGR flow
Exhaust gas recirculation system clogged. Causes: dirty EGR valve (free cleaning if you're comfortable), carbon-blocked EGR passage. Cost: $0-400 depending on complexity.
P0442 + P0455 + P0456 together
General EVAP problem. 60% of the time it's the gas cap. Try that before paying for a diagnosis.
P0500 — Vehicle speed sensor
Sensor that measures wheel speed. Can affect transmission, ABS, speedometer. Replacement: $50-200.
P0700 — Transmission general fault
Generic code indicating another transmission code (specific P07xx) is also present. A more advanced scanner can read transmission codes that cheap scanners miss.
P0011 — Camshaft timing (bank 1, intake)
Variable Valve Timing issue — stretched timing chain, VVT solenoid, old oil. On some engines (notably Toyota), an oil change with the right grade often fixes it.
P0340 — Camshaft position sensor
The sensor that tells the ECU where the camshaft is. Replacement: $30-100 for the part, often DIY.
Clearing a code vs fixing the problem
You can clear a code with any OBD2 scanner. But if you didn't fix the cause, it comes back. Sometimes within 50 km, sometimes 3 000 km. Only clear a code:
- After actually fixing the cause
- To confirm it does or doesn't come back
- Before an emissions test (and even then, the system sees it's fresh — takes a few drive cycles to validate)
Buying an OBD2 scanner
For personal use, a $25-50 scanner (Ancel AD310, Autel AL319, BAFX Bluetooth) does the job. Reads and clears generic codes. For manufacturer-specific codes (e.g., transmission) and live data, you need a $150-400 scanner (Autel MP408, BlueDriver Bluetooth).
Need parts after diagnosing?
Once you know what you're looking for — plugs, coils, O2 sensor, thermostat, gas cap, MAF sensor — we can almost always get the part fast. Call us with your make / model / year and the code. See also our auto parts in Laval and our noise-based diagnostic guide.