The Check Engine Light (CEL) is a warning indicator—it means your vehicle's computer has determined that a component or system in your emission control system is not working properly.
When the light comes on, one or more diagnostic trouble codes (DTC) are stored in the engine control module. These DTCs remain even if the light goes out. To address a Check Engine Light problem, the DTCs are retrieved and the appropriate troubleshooting information is followed in order to determine the problem.
Every vehicle manufactured in the U.S. has to first pass an Environmental Protection Agency (EPA) test called the Federal Test Procedure. This sets the acceptable limits of wear and/or failure for the emission control system—i.e., what conditions will 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. (Note: 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.
The Four Most Common Check Engine Light Scenarios and What to Do
The Check Engine Light turns on and off or flickers.
If the Check Engine Light comes on in the city but goes off on the freeway, then the fault is present during city driving conditions. Pay attention to whether or not the vehicle runs or drives any differently when the Check Engine Light illuminates. If vehicle performance does change, drive the car as little as possible and take it to be checked by a service professional as soon as possible. If there is no change in vehicle performance, you can drive home, but have it inspected as soon as possible. In this condition, you run a risk of the vehicle dying or not starting.
The Check Engine Light comes on and stays on.
If the Check Engine Light illuminates constantly during driving with no noticeable driving or performance problems, there is a permanent fault in the emission control system. When this happens, the computer that controls the emission system usually has a backup program that runs while the fault is present. (These backup programs are often referred to as "limp home" mode programs.) You should get the vehicle serviced as soon as possible, but in most cases, the vehicle will continue to operate, though you run a risk of it dying or not starting.
The Check Engine Light illuminates, stays on, and there are performance problems.
This means that a vital component of your emission control and engine management system has a serious problem. It usually involves a component or system needed for the vehicle to run at all. In most cases, drive the vehicle as little as possible. In many cases, the vehicle is not safe to drive at all—it could stop or stall out at any moment. It is best to pull over to a safe place and have the vehicle towed to an automotive diagnostician for a thorough inspection and repair.
The Check Engine Light light comes on and blinks in a steady pattern while driving.
Don't confuse this steady pulsing of the Check Engine Light light (usually one or more flashes per second) with a flicker (see above). The Check Engine Light may stay on steadily or it may flash when the vehicle is accelerated. This is very serious. There is a severe failure of the emission control system that is causing the engine to misfire to the point that the catalytic converter is damaged each time the Check Engine Light flashes. It may mean that the catalytic converter is overheating to the point that it will glow red or, in extreme cases, start a fire on the underside of the vehicle. Immediately pull over to a safe place and have your vehicle towed to an automotive diagnostician for repair. Vehicles can be severely damaged and even destroyed by fire if this condition is ignored for too long.