The check engine light is a warning indicator: it means your car's computer has determined that a component or system in your emission controls is not working properly.

Every vehicle manufactured to be sold in the U.S. has to pass an Environmental Protection Agency (EPA) test procedure called the Federal Test Procedure. This sets the acceptable limits of wear and/or failure for the emission-control system--i.e., what conditions ultimately cause a Check Engine Light to illuminate. These standards are closely regulated. If the emission-control system is faulty and the vehicle is polluting the air, the Check Engine Light illuminates to alert the driver of this condition. (A vehicle in this condition would fail an emissions inspection or smog check.)

Don't confuse the Check Engine Light with the maintenance or service light. These lights illuminate when a routine service is due. They are usually triggered by mileage, gallons of gasoline consumed, or some other type of vehicle-use measurement.