What your Check Engine Light means.

The "check engine" light is part of your car's 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," "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.

Manufacturers originally used the OBD system to help technicians pinpoint and troubleshoot malfunctions. But the systems now are required under federal laws governing automotive emissions. The 'check engine' light is reserved only for powertrain problems that could have an impact on the emissions systems. Exactly what the OBD system looks for depends on the make, model and year. The original systems varied widely in their capabilities. Some did little more than check whether the various electronic sensors and actuators were hooked up and working.

That changed by 1996, when, under OBD II regulations, carmakers were required to install a much more sophisticated system that essentially acts like a built-in state emissions testing station. 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.

Some states are considering an advanced OBD system that would allow them to do away with emissions testing. If the "check engine" light comes on, the system automatically would send a remote signal to state officials, who would contact motorists who don't have the problem corrected within a reasonable amount of time.

What to do

If the "check engine" light illuminates, it will either blink or remain constant, depending on the problem. Either way, you should have the vehicle checked by a mechanic, although a blinking light or, on some cars, a red light instead of a yellow/orange light indicates a problem that needs immediate attention. In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter, requiring an expensive repair. If that happens, you should reduce power and have the car or truck looked at as soon as possible. If the light is steady, the problem is not an emergency, but you should schedule an appointment as soon as possible. Today's automotive computers often try to compensate when there's a problem; so you may not notice deterioration in performance, even though your fuel mileage is suffering and your vehicle is emitting unacceptable levels of hydrocarbons and other pollutants.

"The customer is really, in the long run, potentially hurting their pocket book by leaving that light on and ignoring it" In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage.

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.

Reduce speed and load. If the "check engine" light is blinking or you notice any serious performance problems, such as a loss of power, reduce your speed and try to reduce the load on the engine. For example, it would be a good idea to stop towing a trailer. Have the car checked as soon as possible to prevent expensive damage.

Don't go for a state emissions test. In a late-model car, an illuminated "check engine" light probably is a sure sign your car will fail the test. In some states, it's an automatic failure, even if the problem was nothing more than a loose gas cap. By the way, don't bother trying to fool the inspection station by 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 inspection station know that its codes have been erased, and you'll just have to go back again.