Does a check-engine light really mean something is wrong with your engine? An OBD2 scanner can tell you. What you should do if your idiot light comes on.

The check-engine light is a signal that the onboard diagnostics system (or OBD II) has detected a malfunction in the vehicle's emissions, ignition or fuel systems.

Consumer Reports explains what the check engine light means and what to do when you see it: how to tell if your car has a loose gas cap—or a serious engine problem.