What’s Up When the “Check Engine” Light Comes On?

by Don Elliott on December 1, 2011

Every car built since the early 1980s has some version of a check engine light on the dashboard when there is a problem with your cars engine and emissions system. The original check engine lights were meant to tell you that a system failure was imminent. If you were stuck along the side of the road with a blown engine, the check engine light may not have been helpful.

Onboard diagnostic systems (OBD) varied widely when first introduced. As the engine’s computer gained sophistication, more systems were controlled and monitored, specifically the emissions control system. In 1996, OBD II established standards mandatory for all cars sold in the United States. Most importantly, standards were established by the Society of Automotive Engineers for diagnostic trouble codes. The regulation also mandated a standard diagnostic connector for a scan tool to read the trouble codes.

Your car’s computer is continually making changes to maximize the efficiency and performance of the engine. Any time the engine’s emissions output exceeds the government-mandated standards for emissions; the check engine light will turn on to indicate a problem. The car’s computer records a trouble code describing the problem. However, it does not necessarily describe what caused the problem.

For example, if the gas cap is loose or missing, the check engine light will come on to indicate a problem. An electronic scan tool or diagnostic computer, standard equipment for a qualified mechanic, will indicate that gasoline fumes are leaking at the gas cap area. However, it will not say whether the gas cap is missing, loose, or damaged in some way that it doesn’t seal properly. A physical inspection is required to figure out why the check engine notification is lit.

If the check engine light comes on and blinks in a steady pattern or glows red, it is an indication of a serious problem that requires immediate attention. Continuing to drive the car could result in damage to the engine, catalytic converter, or other major components. Pull over at a safe location and turn the car off as soon as possible. It will be necessary to have your car towed to your auto service center.

If the check engine light stays on or turns on and off intermittently, it is an indication that there is some sort of performance problem causing emission readings outside of preset parameters. In most cases, the car is safe to drive until the specific trouble code can be identified and the problem corrected. If the car was not performing well when the light came on, then the problem is more serious.

Some auto parts stores will do a simple diagnostic readout for no charge. Or you can buy a basic scan tool for about $50. The scan tool connection is located within two feet of the steering column inside of the driver’s compartment. As mentioned earlier, reading the trouble code is only the first step. Discovering what caused the check engine light, the bad reading, and how to fix the problem is the job of a good qualified mechanic.

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