Why is my check engine light on?

A solid check engine light means the car's computer (PCM) detected an emissions or drivetrain fault and stored an OBD2 trouble code. The most common causes are a loose gas cap (P0455/P0457), oxygen sensor (P0135/P0141), catalytic converter (P0420), or misfire (P0300). A flashing check engine light means an active misfire — stop driving and get it towed.

Cost range: $0–$2,500. Severity: medium. Safe to drive? Solid light: drive carefully and scan within a few days. Flashing light: do not drive — an active misfire dumps raw fuel into the catalytic converter and can destroy it ($1,500+).

Most likely causes

1. Loose, damaged or missing gas cap — Triggers EVAP codes (P0455, P0457, P0442). Tighten until it clicks 3 times — the light may take 50–100 miles to clear on its own. ($0–$30, common)
2. Failing oxygen (O2) sensor — P0130–P0167 family. Sensors degrade after 80–120k miles, hurting fuel economy and emissions. ($150–$500, common)
3. Catalytic converter efficiency low — P0420 / P0430. The cat is no longer cleaning exhaust effectively. Aftermarket replacement is much cheaper than OEM. ($200–$2,500, common)
4. Misfire (worn spark plugs / coils) — P0300, P0301–P0308. Causes flashing CEL when active. Plugs and coils are usually under $200 in parts. ($50–$1,200, common)
5. Mass airflow (MAF) sensor or vacuum leak — P0171/P0174 lean codes or P0101–P0103. Cleaning the MAF with electronics cleaner often fixes it for free. ($0–$400, common)