You're driving along in your car or truck and suddenly a yellow light illuminates on your dash telling you to check or service your engine. If you're like most car owners, you have little idea about what that light is trying to tell you or exactly how you should react.

Call it the most misunderstood indicator on your dashboard, the "check engine" light can mean many different things, from a loose gas cap to a seriously misfiring engine.

"It doesn't mean you have to pull the car over to the side of the road and call a tow truck. It does mean you should get the car checked out as soon as possible, but ignore the warning, and you could end up damaging expensive components. It also can be a sign that your car is getting poor fuel economy and emitting higher levels of pollutants.

What the Light Means

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.

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 to pinpoint and troubleshoot malfunctions.

The computer monitors and adjusts dozens of components and processes. For example, it continually samples exhaust emissions as they come out of the engine and again when they leave the catalytic converter, a device that removes carbon monoxide and hydrocarbon pollutants from the exhaust. The system also monitors your car's fuel system to ensure that gasoline vapors are not escaping into the atmosphere through a leak or even a loose or missing gas cap. In most cases, if a problem occurs, the computer will wait to see if it corrects itself before turning on the light. Modern OBD II systems are so thorough that state testing centers increasingly are checking for any stored trouble codes and foregoing the traditional tailpipe emissions test.

If the check-engine light comes on, here are some tips on what you should do:

• Look for a serious problem that requires immediate attention. Check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so. On some cars, a yellow "check engine" means investigate the problem, while a red "check engine" means stop right now.

• Try tightening your gas cap. This often solves the problem. Keep in mind that it may take several trips before the light resets. Some vehicles have a separate indicator that warns of a loose gas cap before the condition sets off the "check engine" light.

• Have the code read and the problem fixed. If you are due for a state emissions test. In a late-model car, an illuminated "check engine" light probably is a sure sign your car will fail, it's an automatic failure, even if the problem was nothing more than a loose gas cap. By the way, don't bother disconnecting the battery or using any other method to erase the trouble code and turn off the "check engine" light. Your vehicle's computer will let the states inspection computer know that its codes have been erased. The monitors for the emission system components will go to a not ready mode stopping from issuing a inspection certificate.