The "check engine" light is part of your car's so-called onboard diagnostics (OBD) system. Since the 1980s, computers increasingly have controlled and monitored vehicle performance, regulating such variables as engine speed (RPM), fuel mixture, and ignition timing. In some cars, the computer also tells the automatic transmission when to shift.
Experts say that many drivers confuse the "service required" light on the gauge cluster for the Check Engine light. These warning lights are unrelated. The service required light just means the car is due for an oil change or other routine maintenance. It is not the indicator of trouble that the Check Engine light is.
Check Engine lights come in orange, yellow or amber, depending on the manufacturer. If the light begins flashing, however, it indicates a more serious problem, such as a misfire that can quickly overheat the catalytic converter. When the light is flashing, get the vehicle to a repair shop as quickly as possible. Continuing to drive the vehicle could cause more damage to the affected system. These emissions devices operate at high temperatures to cut emissions.
When it finds a problem in the electronic-control system that it can't correct, the computer turns on a yellow warning indicator that's labeled "check engine," "service engine soon" or "check powertrain." Or the light may be nothing more than a picture of an engine, known as the International Check Engine Symbol, perhaps with the word "Check." In addition to turning on the light, the computer stores a "trouble code" in its memory that identifies the source of the problem, such as a malfunctioning sensor or a misfiring engine. The code can be read with an electronic scan tool or a diagnostic computer, standard equipment in auto repair shops. When a repair shop reads a code from your vehicle, the code tells the technician where to start looking and testing. Most of the time, it takes some diagnosis time to determine what caused the light to come on.