The mystery of the ‘check engine’ light

But this is where a little knowledge can be a dangerous thing, Jaap cautions. The readings are often vague, and won't necessarily tell you what needs doing.

"It can be even more expensive to try and guess what's going to fix it, just based on what a scan at an auto-parts store tells you," Jaap says. "That's kind of hearsay and conjecture, and I'd go broke if I did that."

At the very least, the code can help you (or your mechanic) decide whether it's something that can be put off for a bit.

Can't my car just tell me what's wrong?
Said Deep, a spokesman for Ford Motor Co., says indicators on modern cars are getting more sophisticated, telling drivers, for instance, how close they are to needing an oil change, or when a tire is getting low.

"But there's a limited amount of real estate on the instrument panel," Deep says, "and there's a certain amount of information overload that we want to avoid."

Carmakers are only adding messages that drivers are comfortable dealing with — specific messages for a headlight out, say, or low washer fluid.

"When we get into computer operations, like delicate air/gas mixture issues or the way your transmission shifts, that's something we think is more suited for a trained mechanic with the proper equipment," Deep says.

So what's the light saying?
The computer can detect scores of possible problems, ranging from silly to sinister. As often as not, the MIL comes on when the gas cap isn't screwed on tight enough or the oil dipstick isn't pushed in all the way.

But let's say the computer indicates a malfunctioning oxygen sensor, an often troublesome doodad that measures the amount of oxygen in your exhaust. Without its feedback, the engine will surely start, run and seem to perform normally. Fuel economy may dip a bit, though you'll hardly notice. Why bother with a costly repair?

Because when it comes time to pass a state emissions inspection, forget it. If the exhaust coming out of your tailpipe isn't just so, no license plate for you.

At its core, that's really what gave rise to the MIL in the first place.

How it got so complicated
Before the 1980s, the dashboard gave gearheads all the vital signs they needed to keep moving down the road: fuel level, oil pressure, temperature, RPMs and, in some cases, alternator voltage.

But these dials revealed nothing about the intricate balances of fuel mixture or carbon emissions. As smog became a political issue in the early 1980s, the federal government intervened with emissions control standards. Making a car perform up to snuff got a lot more complicated.

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There are endless ways our cars can run afoul of these standards; without sensors to point the way, mechanics would spend more time diagnosing than fixing. And other than an annual emissions inspection, how would Earth-friendly drivers know their vehicle was an environmental menace?

In that vein, perhaps it's only fair to think of the MIL as your friend — the automotive equivalent of a canary in the coal mine.

"Older cars were simpler, but we've come a long way," Jaap says. "I have to say that it's really easier to diagnose the modern engine - if you have the right tools. Its an all-around better world."

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