There are few things that strike more fear into the heart of a conscientious driver than the ominous 'check engine' light. Since it tells you nothing about what the problem is, do you need to pull over on the highway and call a tow truck? Or can you get to a service station? And what does it mean, anyway?

If your car hasn't stalled and isn't smoking, it is probably safe to drive it to the nearest service station or auto parts store. As you drive, pay attention to the way your car looks, sounds and handles. If you see smoke, find it difficult to steer or brake, or hear strange noises, pull over and call for help.

Once you get to the mechanic or auto parts store, they will run a diagnostic test to tell you why the light came on and what can be done to fix it.

The most common reasons for the check engine light to come on are:

The gas cap wasn't replaced properly, causing fuel vapors to leak from your car. This is common, and the easiest and cheapest cause to remedy. If you just filled up, check this first. You may not have tightened the cap all the way, or it may have gotten cracked or damaged. Remove the gas cap, inspect it for damage, and then carefully replace it. If the gas cap is damaged, you can buy a new one for a few dollars at an auto parts store.

The spark plugs are worn, and misfiring. This will cause your car to shudder when you accelerate, so if you feel that, check the spark plugs. Spark plugs usually last about 100,000 miles, but even if your car is fairly new, they could be faulty. Replacing them is fairly straightforward. Your mechanic can do it quickly or you could even do it yourself. Ignoring this can lead to further damage, though, so address this as soon as you can.

An oxygen sensor has failed. Oxygen sensors measure the amount of unburned oxygen to keep fuel burning efficiently. The sensors get dirty over time, and can stop working. This will lead to a decrease in gas mileage and more harmful emissions. This is a fairly easy fix and the process may be explained in your owner's manual. Don't ignore it, though. If you don't replace the failed sensor, it can damage your catalytic converter and then you're looking at a big repair bill.

The catalytic converter needs to be replaced. If your check engine light came on due to worn spark plugs or a failed oxygen sensor and you ignored it, you may damage your catalytic converter, the part of your car that reduces the gases in your exhaust. If your gas mileage falls, or your car doesn't accelerate properly, the catalytic converter may be at fault. This requires a trip to the mechanic, and may cost a few hundred to a thousand dollars to fix. If you ignore it, eventually your car will stop running.

Most things that cause the check engine light to come on are relatively inexpensive and easy fixes if they are addressed right away. While it's usually not necessary to pull over on the highway and call a tow truck when the light comes on, addressing the problem at the first opportunity will prevent further problems down the road. (For more information, contact American Transmission Center)