The mystery of the ‘check engine’ light

updated 12:39 p.m. ET Sept. 21, 2007

It's just a little amber lamp in the dashboard, a tiny bulb behind a plastic pane the size of a postage stamp, inscribed with a very big message.

Depending on the car's make and model, it might say "CHECK ENGINE," as if you possessed the ability to pry open the hood and make a snap diagnosis. Others say "SERVICE ENGINE SOON," worded just vaguely enough to enable us procrastinators.

No matter how it's communicated, the effect can be unnerving. Who knows what evil lurks in the hearts of engines? Only the computer knows.

But unraveling the mystery of the enigmatic little lamp is considerably easier — and usually less painful — than most people think. While there's little reason to shut the vehicle off and sprint for the nearest mechanic, the light shouldn't be ignored for long.

"It's your car's way of telling you, 'You might be meeting Larry the tow-truck guy soon,'" says Bill Jaap, owner of Good Carma VW and Audi Repair in Minneapolis. "It's a warning, and it can be menacing. Even though the car still runs well, I think it's a good idea to bring it in."

What it is
The lamp's formal name is the malfunction indication light, or MIL.

Today's cars are equipped with dozens of sensors and a computer that make on-the-fly adjustments to maximize fuel efficiency and minimize carbon emissions. If a sensor detects something amiss, it sends a code to the computer, and voila: "CHECK ENGINE."

That code is stored in the computer — and the light stays on — until it's retrieved. And that part is surprisingly simple: It's just a matter of plugging a handheld gadget into a connector that's usually located beneath the steering column.

What not to do
Panic. Unless your car is idling rough, starting hard, hesitating or smoking, it's OK to keep driving when the light comes on. (In a few models, the light will flash or turn red if the problem is serious. If this happens, DO pull over ASAP.)

But it's unwise to ignore it. For one thing, the problem will likely mean a failed emissions inspection down the line. For another, many of these small, undetectable problems can cause costly engine damage if left untreated.

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"It's kind of a false signal to some people who maybe have been driving around with it on for a year," Jaap says. "Even though the problem can be very slight, my opinion is that you shouldn't get desensitized to it. It's on for a reason."

Tip: Don't pay a mechanic to read the code unless the shop offers to credit the money back in related repairs. The service is easy enough to get for free.

What to do
Obviously, if the car is under warranty, take it to the dealer. But with older cars, many mechanics and auto-parts stores will retrieve the code gratis.

Do-it-yourselfers can purchase code readers for $50 to $200, and some software programs will turn your laptop into a scan tool with a special connector. Some readers give a number that needs to be cross-referenced; more expensive scanners will display details.

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