Federal regulations required all vehicle manufacturers to have certain core trouble codes in common with their computer (EEC) controlled vehicles. These standards include universal data interfaces, trouble codes, as well as standard component terminology. These standards are known as OBD-I and OBD-II (OBD stands for On-Board Diganostics). Due to this standard, many auto parts stores read out the OBD trouble codes free of charge (in hopes they can sell you the necessary parts for your auto repair job).

THE CHECK ENGINE LIGHT
Located on the instrument panel of all modern cars, the "Check Engine Light", also sometimes called the, "Service Engine Soon" light or more technically known as the Malfunction Indicator Light or lamp (MIL) is used to inform the driver of a failure in the engine or transmission control system that could affect vehicle emissions. When this light comes on it does not automatically mean that you are facing a huge auto repair bill. It could simply indicate a glitch in one of the sensors that feeds the electronic engine control unit (EEC). This light is an output of the EEC control module and, prior to 1996, is used to display diagnostic trouble codes which aid a mechanic in carrying out an automotive diagnostic and repair procedure. From 1996 and on, the trouble codes (called OBD-II or just OBD2) were standardized on all vehicles sold in the United States. There are about 500 generic OBD-II codes as well as dozens of manufacturer specific trouble codes (Ford, GM, Chyrsler, etc).

The MIL is located in or near the instrument cluster. It will illuminate, as a bulb check, when the key is turned to the run position with the engine off. After the engine is started, the lamp will remain on for a few seconds then go out. If the engine control module detects a failure in the emission system, the lamp will be illuminated to inform the driver of the need for service in which case you should visit your nearest auto repair shop for analysis.

Government regulations have affected the operation of the check engine light as of 1996. OBD-II regulations require that the lamp illuminate when a failure in the vehicle emissions system causes the vehicle to exceed one and a half times the federal emission requirements. In addition, the vehicle MIL must flash to warn the driver of a catalyst damaging misfire.

INTERMITTENT CHECK ENGINE LIGHT
An intermittent check engine light situation is often very difficult to diagnose since the light will go on and off thus making it hard or impossible to capture the underlying trouble code. On some cars and trucks the check engine light will go off when the problem self-corrects or goes away (eg. one of the sensors now measures a normal reading). On others you'll need to use a scan tool to turn the light off manually but, of course, only after the necessary auto repairs have been performed. Still on other cars, the underlying problem (if there even was one) may self-correct and yet the check engine light will still stay on; this requires it to be manually cleared. Due to strict California emission standards, some vehicles may set a evaporative emission OBD-II trouble code even if you leave the gas cap loose. If the check engine light light goes out, the OBD-II code usually will still be stored in the EEC computer as long as the battery in the car stays connected. But on some of the newer vehicles, the OBD-II codes stay in memory even if you disconnect the car battery.