When your car's "Check Engine" light comes on, it's usually accompanied by a sinking feeling in the pit of your stomach. The light could mean a costly problem, like a bad catalytic converter, or it could be something minor, like a loose gas cap. But in many cases, it means at minimum that you'll be visiting the car dealer to locate the malfunction and get the light turned off.

The Check Engine light â€" more formally known as the Malfunction Indicator Lamp (MIL) â€" is a signal from the car's engine computer that something is wrong. The car dealer's service department can diagnose the problem for about $75. But there's a way to preview what the problem might be.

Prior to 1996, carmakers had their own engine diagnostic systems, primarily to ensure their cars were compliant with Environmental Protection Agency pollution-control requirements Autel MaxiSys MS906TS. Starting with model-year 1996, automakers standardized their systems under a protocol called OBD-II, which stipulated a standardized list of diagnostic trouble codes (DTC) and mandated that all cars provide a universal connector to access this information. It's usually located under the steering column and is easy to access.

CarMD isn't alone in the code-reader market. An Internet search will bring up countless devices, some costing as little as $40. Most come with a booklet listing the codes, but it is also easy to do a Google search to locate the codes. Aamco will check the Check Engine light for free and provides a fact sheet.

As Dan Edmunds, director of vehicle testing for Edmunds.com, points out, the system is primarily designed to continuously monitor a car's emissions system over the life of the car. However, he notes, "The engine and the emission control system are so interlinked that the health of the emission control system is a good indication of the general health of the car's engine Autel MaxiSys Pro."

Steve Mazor, the Auto Club of Southern California's chief automotive engineer, says that while some people freak out when they see the Check Engine light, "others just put a piece of black tape over it and keep driving." Mazor says it's important to promptly address problems indicated by the light. Ignoring them could lead to larger, more costly problems later.

If the light comes on, Mazor says the driver should first see if the gas cap is loose: That's a common cause. A loose cap sends an error message to the car's computer, reporting a leak in the vapor recovery system, which is one aspect of a car's emissions system. If the gas cap is loose, tighten it and continue driving. Even so, it will take some time for the light to go off, he says.

Mazor says that even an inexpensive code reader could be useful for car owners, even if they aren't mechanically inclined.

"If the mechanic gives you the same information, at least you know they are going down the right road," he notes. Edmunds agrees, adding that a code reader provides car owners with one more data point to help them talk with their mechanic and avoid costly or unnecessary auto repairs.

Mixed Signals
But even with the code and its meaning in hand, do-it-yourself interpretation can be a little tricky â€" even if you are mechanically inclined, as Dan Edmunds explains.

"My wife's car started running poorly and there was a Check Engine light. My code reader detected a code for the Cam Angle Sensor. I thought about buying the sensor and installing it myself, but if I had, I would have wasted time and money because it turned out that the sensor was fine. Instead, mice had gotten under the hood and had chewed some of the wires leading to it."

Occasionally, the Check Engine light comes on when nothing is wrong with the car, Mazor says. It could be a temporary problem caused by a change in humidity or other factors. In such cases, the light should go off by itself after a short time.


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