Trouble Codes

Just got a check engine light? Start here.

Before you look up a single code, run through the triage order: steady vs. flashing light, what to write down, how to get the code read free, and when it's safe to keep driving. Read the check engine light diagnosis primer.

What is an OBD-II trouble code?

Since 1996, every car sold in the US has carried an On-Board Diagnostics (OBD-II) system that constantly self-tests the engine, transmission, and emissions equipment. When a test fails, the computer stores a trouble code — a letter followed by four digits, like P0300 — and turns on the check engine light. The code tells you which system failed its test, not which part to buy. That distinction is the whole game: diagnosis first, parts second.

Each code below has its own page with the meaning, how urgent it is, the most common causes in order, and the diagnosis sequence — plus model-specific pages with repair guides, parts, and costs for your exact vehicle. Pick your code to begin.