What does the check engine light actually mean?

The light means the engine computer logged a fault code, and the code is the whole story, so get it read: auto parts chains do it free in minutes, or a $20 Bluetooth OBD2 plug reads it to your phone. The single most common trivial cause is a loose gas cap, which breaks the fuel system's pressure seal; tighten it and the light may clear itself over a few drives. Beyond that, the frequent offenders are oxygen sensors, catalytic converters, and ignition components.

The critical distinction is steady versus flashing. Steady means diagnose soon, not panic. Flashing means an active misfire is dumping unburned fuel into the catalytic converter and cooking it, a four-figure part, so ease off, and get the car looked at immediately.

Do not just clear the code and hope; it returns if the fault is real, and the light also fails you at emissions inspection. Reading the code first also armors you against the mechanic's mystery-diagnosis markup.