Last updateWed, 18 Oct 2017 10am

On The Road

When 'check engine' light goes on, see technician for diagnosis

An older man came into the shop the other day asking for help, saying that his "check engine" light was on. He opened the hood to check the engine, then realized that he didn't know what exactly he was supposed to be checking. The oil? The coolant? What was that light trying to tell him?

"I need another hint," he said.

Though it has more names than the devil himself - including "malfunction indicator lamp," "service engine soon," and "emissions workshop" - this ambiguous little light is most commonly referred to as the "check engine lamp."

Perhaps one of the most mysterious and confusing events that can happen to a driver is seeing that nebulous check engine light turn on. What exactly does it mean and what are you supposed to do?

This warning lamp first began appearing on cars in 1980 in order to meet federal emissions standards. It was later required on all vehicles to inform drivers that their vehicles may be producing more than the allowable level of emissions.

Today, however, the check engine light can warn you of a wide range of potential problems, from letting you know about elevated emissions levels to warning you that damage to the engine may be occurring. It can even warn you of a problem that might cause the engine to fail before it happens, thus saving you the inconvenience of being stranded on the side of the road.

The computer that controls the engine in vehicles built in the last 25 years is constantly monitoring the performance of areas such as ignition, fuel and emissions as part of its on-board diagnostics. When the computer detects something that isn't working properly, it stores a code in its memory and turns on the check engine lamp to inform the driver that there is a problem.

Today's engine computers can have several hundred different codes, and that means several hundred reasons for the light turning itself on. Anything from a simple loose gas cap to a serious mechanical engine problem could be the cause.

How can you find out if it's a serious problem or not? Your technician can check which code is turning the light on by "pulling the codes" or scanning the on-board computer using a variety of specialized computer scan tools. Once this is done, the scan tool will give a code number that can give a better idea about the nature of the problem and its seriousness.

The code will only give a general idea about the problem. To determine the specific problem, which part(s) may be affected, the time required and the cost to repair, additional diagnosis is often required. The technician must start checking sensors, wires, vacuum hoses and other components to determine the exact nature of the failure.

So when that check engine lamp comes on, take your vehicle to a trained technician with the right tools and tell him, "I need another hint."

John Polstra operates European Auto Works, 239 W. Evelyn Ave., Mountain View, with Cliff Greenman and Ralph Foglein. Send questions to them at This email address is being protected from spambots. You need JavaScript enabled to view it..

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