What's wrong when the check engine light is.

When that light turns on there is a diagnostic fault code stored in the Engine Computer. That code will indicate the circuit or system that needs further diagnosis.

The level of severity can be determined by how that lights acts. Flashing is the most severe and rare. It means too much raw unburned fuel is going into the exhaust system and is going to overheat and damage the catalytic converter. That's an expensive repair and you are supposed to stop the engine right away.