Such a system is in place on all vehicles built after 1996. The check engine light is just that, an early-warning system. Unfortunately, this device is almost completely misunderstood and often seen as an enemy, rather than a friend. When the light comes on, a situation exists that will become a problem. Ignoring the light will always mean increased expense, in time.

Human nature is such, that people try to postpone or even ignore things they do not understand. This is particularly more acute when the possibility for unpleasantness exists. Vehicle problems fall squarely into this category. They are almost never expected, and the costs can be considerable. A bit of knowledge can help. Learning how the warning system operates can relieve much of the anxiety.

Over two-thousand different functions are monitored by the check engine light, and a new "drive-cycle" is initiated each time the vehicle is started. When a problem is seen, in a drive-cycle, the light is illuminated and a diagnostic trouble code or DTC is placed in memory. The light will stay on when the condition is active or until the drive-cycle ends.

Not every test can be run during each drive-cycle, as some require specific conditions. If the problem component is not tested, the light will go out. This does not mean the problem is corrected, only that the tests have not run. The DTC will remain in history, and the problem can still be found.

For instance, to test a fuel-air sensor, the engine must be fully cooled. This means that the engine coolant temperature must be less than the ambient temperature. Starting the engine begins the test sequence. If the driving distance is adequate, the test will be completed. On short trips, there may not be time to complete the tests. Here, the test will abort. Many hours are required for engine temperature to fall below outside air temperature. The tests will likely not run again that day, and the light may go out.

The next time the conditions are met, the tests will execute and the light will come back. If this occurs infrequently, as with a vehicle driven for short distances, the driver may ignore the warning. This is a big mistake. A small problem, such as a bad fuel pressure regulator, can set a fuel-air sensor code. Ignored, the rich fuel condition may damage the catalytic converters and cost several thousand dollars to repair. Check engine lights are an early-warning system.

Folks often think the code stored tells what is wrong with the vehicle. This is not true. The DTC suggests an interpretation of data the computer receives. Many things can set any given code. For example, the fuel-air sensor was showing a reading that was out of range. The sensor was not the problem. A leaking fuel pressure regulator caused the mixture to be out of range, and the sensor reports the condition. Replacing the sensor is a waste of money and will not help the problem.

Trying to guess the cause of a problem will be a very expensive and frustrating proposition. Trained professionals use several methods and pieces of equipment to learn the cause, before any parts are replaced.

A common misconception is there is a machine that tells what is wrong. Determining the cause of a check engine light is highly skilled work, requiring knowledge of the system. Many techniques and pieces of equipment are used to find the problem. A thermometer will tell if a patient has a fever. Elevated temperature is not the problem, rather a result of another issue. The doctor may need to test white blood cell count, blood pressure and several other things to learn the cause. This holds true with a check engine light as well. The stored code only tells that a fault has occurred, not why, where or what is required to correct it.

Quality shops will always provide the DTC they are addressing and what is required to correct the issue. An invoice from the shop should list all codes found. Should the light come back, knowing the previous DTC will tell whether it is a new problem or the same. A diagnostic trouble code will start with a letter, such as "P." This letter shows the computer that logged the problem. The power control module uses "P."

Numbers following the letter give more information. The code P0301 suggests a misfire on the cylinder number-one. ‘P’ means the power control module; the "03" is a misfire code, and the ending "01" is the cylinder affected. Should the light come back, with code P0420, there is a different problem.

The least-expensive route is quick attention by a trained professional. Very good diagnostic technicians are quite rare. Shops that do not employ these highly paid individuals, normally sell menu items. A list of services, with prices will be prominently displayed in such establishments. For example, generic terms like "brake job $79.99" and "Tune up $99.99" are a tip off. Selling menu services leaves the diagnosis to the client.

The menu approach will cost a huge amount and not likely solve the problem. A sure tip is to ask, "What is wrong and is the solution guaranteed to correct the problem?" From the answer, it can be judged if the correct shop has been chosen.

Diagnosis is the most difficult and expensive service a shop offers. "Free checks" are simply that, a bid to get someone in and sell items. The best shops will have an established rate per hour and charge only for the time they spend. Set fees are not as desirable, as they cover worst case conditions. Minimum charges are also not fair, when five-minutes are required to isolate a loose wire.

Limits can be established beforehand. For instance, do not exceed two hours without my consent. When dealing with an honorable business, this is largely unnecessary. If there is no experience using the shop, it may be a wise precaution. The least-expensive diagnosis will always come from the most trained individual.

  Act promptly, do not let the light go unattended.

  Use a professional for diagnosis.

  Record and retain the codes that are being addressed.

  Ask for an explanation of what will be done to correct the problem.