A check engine light usually indicates a problem with a vehicle's emissions control system.
A check engine light usually indicates a problem with a vehicle’s emissions control system.

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By Jim Kerr

In decades past, we discovered our vehicles were not operating correctly by the symptoms they produced: the engine would take a long time to start, or perhaps it wouldn’t start at all. Transmissions would grind or be difficult to shift, and problems with the electrical system meant something simply didn’t work.

Today, we still use symptoms to diagnose some problems, but many minor problems are compensated for by the vehicle’s computers and the only indication we get of a problem is when a warning message or light appears on the dash. So what should we do when that dreaded light appears?

The function of the “Check-engine Light” (CEL), sometimes called the “Service Engine Soon Light,” and by regulations officially called the “Malfunction Indicator Light” (MIL), has changed over the years. Prior to 1996, the CEL was turned on by the fuel injection computer to indicate a problem with any computer-controlled system on the car. On-board diagnostics were still relatively in their infancy and a fault such as a broken wire or shorted solenoid would cause the light to turn on immediately. As soon as the problem was repaired, the light would go out. Along with the warning light, diagnostic codes were stored in the computer memory to help technicians find the problem.

In 1996, OBD II (On Board Diagnostics level 2) were mandated by Federal emissions regulations and this applied to all passenger cars. Light duty trucks were not required to comply until 1998 but many trucks conformed in 1996. These regulations have the Check-engine Light come on only when there is a vehicle problem that fails the diagnostic tests twice and causes vehicle emissions to be excessive.

In addition, the problem must now pass diagnostics three times before the light will go out. Although the sophistication of the on-board vehicle diagnostics has increased dramatically since 1996, the requirements to turn the light on and off are still the same.

So when that CEL comes on, it means the problem has appeared twice and it could take a lot of driving to turn it off. The problem could be caused by anything from a misfiring sparkplug to a leaking gas cap. Because the computer can compensate so well for many problems, the vehicle should be taken in to the repair shop for diagnosis. Otherwise, serious problems could develop. For example, if a cylinder is misfiring partially, this causes the engine to run rich. This wouldn’t be a serious problem if you were driving across town, but drive a few hours on the highway and it could load the engine crankcase with excessive fuel, which could destroy the engine.

Many of the problems that cause the CEL to turn on are minor, but without having a diagnostic scan performed, you might not know. A common problem is what we call “gas cap codes.” This problem turns on the CEL because the fuel system isn’t sealed properly, often because the gas cap isn’t tightened completely. This type of problem won’t cause additional damage if the vehicle is driven, but having a scan performed is the only way to determine this.

The CEL can also flash if a problem is severe. If a failure causes the catalytic converter to overheat, which damages the converter, then the light stays flashing. Damage to the converter can happen in only a few blocks, so if the light is flashing, pull over, stop the engine, wait a minute and restart it. If the light is still flashing, call a tow truck!

For non-emissions-related problems, drivers are usually warned by other lights on the dash or a message in the instrument display. There are some problems such as an overheated engine or a red brake warning light that would indicate that the vehicle should be stopped the vehicle. For most problems, if the vehicle still appears to be operating properly and there are no unusual noises or operation, then it is usually safe to drive carefully to the repair shop.

There are many warning lights on a modern vehicle. They should all come on when you start the vehicle as a bulb check, so spend a few minutes with your owner’s manual and turn the ignition key on to see most of them. Familiarity with them will allow you to make wise choices when one comes on, and that provides peace of mind.

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