What does the check engine light actually mean?

The light means the engine management system has recorded a fault code. It covers hundreds of possible conditions, from trivial to serious, which is why the light alone tells you very little. What matters is how it behaves.

A steady light means a fault has been logged and the car is safe to drive to somewhere it can be checked. A flashing light means something is happening right now that can damage the catalytic converter, typically a serious misfire. That is the one case where you should reduce speed and stop driving promptly, because continuing can turn a moderate repair into an expensive one.

The most common cause by a wide margin is a loose or failed fuel cap. The system monitors the fuel tank for vapour leaks, and a cap that is not sealing registers as one. Tightening it and driving for a day or two often clears the light on its own.

Beyond that, frequent causes include oxygen sensors, ignition coils and spark plugs, mass airflow sensors and catalytic converters. These vary enormously in cost, which is exactly why guessing is unwise.

Reading the code is straightforward and cheap. Every car sold since the mid-1990s in most markets has a standard diagnostic port, inexpensive readers are widely available, and many parts shops read codes free. The code narrows the problem to a system rather than naming the failed part, so it is a starting point rather than a diagnosis.

One caution: clearing the code without fixing the cause simply turns the light off until the fault is detected again, and it also erases readiness data that an emissions test requires, which can mean failing the test for an unrelated reason.