What does it mean when your check engine light comes on?

When your check engine light comes on, it means your vehicle's onboard diagnostic system has detected a problem and stored an error code. It can indicate a wide range of issues, from something minor like a loose gas cap to more serious problems like a failing catalytic converter, oxygen sensor, spark plugs, or engine issues. If the light is steady, you can usually continue driving but should get it checked soon. If it's flashing, that typically signals a more urgent problem, like an engine misfire, and you should reduce speed and have it inspected right away. The only way to know the exact cause is to have a mechanic or auto parts store read the diagnostic trouble code with a scanner.