The Check Engine Light - What it Means and What to Do About It

You may have noticed a yellow warning light on your dash that may say "Service Engine Soon" or simply look like a picture of an engine. This is the Malfunction Indicator Light, or Check Engine Light. If the light is on while you are driving, it indicates an emissions related failure of a system or component on your car.

Keep in mind that emissions standards are pretty tight, and the light can be turned on by literally hundreds of different causes. Sometimes, the light comes on with no noticable change in the car's performance, and sometimes the car performance suffers significantly. Anything from a loose gas cap to a major engine fault can trigger the light.

The way this light works is through your car's on board engine management computer. It has the primary job of making sure the exhaust emissions remains within limits. It runs "tests" on these systems and components... some are run all the time, and some only run once a drive cycle. If a test it runs fails, it stores that information. If it is a critical system, it will turn the light on as well. If not, the computer will wait until a second testing verifies the problem, and then turn the light on.

Finding the problem starts by connecting to your car's computer and asking it what problem it saw. It tells the technician this in the form of a Diagnostic Trouble Code or DTC. A common mistake is to take your car somewhere and having someone not trained retrieve this information for you. For example, you may find a code stored for an oxygen sensor biased lean and be sold a replacement part that may or may not solve the problem. The computer can't tell us (yet) exactly what caused the problem or test failure... only what test it ran. It is up to a trained technician to diagnose the exact cause to insure a proper repair.

If your Check Engine Light comes on, at the very least it is affecting the fuel economy and performance of your car, and should be tested to make sure it's not a major problem. Most reputable technicians will retrieve the code and then recommend a course of action for little or no fee.