Solve the Mystery of a Check Engine Light


Perhaps the most mysterious warning light on your car's dashboard is the "check engine" light, which can illuminate seemingly for no reason whatsoever and lead to an unexpected bill just for diagnosing the cause. However, it is possible to diagnose, and sometimes even fix, the cause of this warning light without paying a mechanic.

Your car's check engine light is linked to its on-board diagnostic system, which measures functions such as engine speed, ignition timing, fuel mixture and sometimes, the timing of gear shifts. It then uses that information to identify problems in those functions that could affect emissions and performance. There is a wide range of problems that will cause the light to illuminate, ranging from minor to major, so it's important to determine the cause promptly.

Before you find yourself in a situation with the check engine light illuminated, read your owner's manual to understand why it may illuminate and if it has different warnings to help you understand whether the problem is minor or more serious. In many cars, a check engine light that flashes rhythmically -- not just flickers on and off -- indicates a severe problem that needs immediate attention, while a steadily illuminated light indicates a less serious problem.

Once you thoroughly understand how your check engine light operates, here are three strategies you can try before paying your mechanic for a diagnosis.

Remove and reinstall your gas cap, and make sure it's tight. Because the check engine light is associated with your car's emissions system, it will recognize if there are fuel vapors leaking from around your gas cap and set off the warning light. A loose or improperly installed gas cap is the most common reason for the light to illuminate. If the light does not go out after checking the gas cap and the car seems to be running fine, don't assume that was not the problem. It can sometimes take a few trips for the system to reset.

Get the engine's diagnostic code from an automotive pro for free. If you own a General Motors vehicle with OnStar, contact an OnStar adviser who will read the code from your car remotely. Many auto parts stores and franchise oil change and transmission-service shops will read the code for free, though they may pressure you to do the repair with them. If you get a code read by one of these shops, ask for the code itself -- usually a single letter, followed by numbers -- and then use the shop for the repair if you are confident of the work.

Get the code yourself. For $50 to $100, you can buy a diagnostic-system code reader at your local auto parts store or online. Some newer system scanners can connect right to your smartphone, such as the GoPoint GL1.

Many cars have some method of displaying the trouble code, usually using the key and either rotating it between the accessory position (the position where you can power the radio and other accessories without the car running) and the "on" position (without starting the car) a specific number of times. With some, you keep the key in the accessory position, and push and hold the trip odometer and reset buttons. To find instructions, do an Internet search on the year, make and model of your car plus the words "check engine code" and "manually."

Once you know the code, you can research what it is associated with on websites such as (for on-board diagnostics codes) or In some cases, you can get instructions on how to repair the car yourself on those websites.

Keep in mind that, if there is a problem beyond a loose gas cap, the trouble code produced by the diagnostic equipment is usually associated with more than one problem. Ultimately, it's good to work with a trusted mechanic who can deduce the cause of the problem in your car.

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