From Matthew Wright,
Your Guide to Auto Repair.
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CHECK ENGINE. There's nothing fun about those two words. There's also not a lot of logic to be gathered from them. Check engine? Could they be a little more specific? Nope, they can't. That's because the Check Engine light comes to life if anything, and we do mean anything isn't 100% under the hood. This means that you could be staring at a major repair, or your gas cap could be too loose (no kidding).

Unfortunately, the majority of recurrent Check Engine episodes eventually lead to some professional repair time. The most common problems that trigger the light are emission control malfunctions. The emission control system is what your car uses to try to keep our air a little cleaner. To do this, it employs dozens of sensors, valves, flaps, heated wires and probably some fairy dust. Every car made in the last 20 years has at least one oxygen sensor (we saw a Toyota that had four of them recently), and they don't last forever. If they go, expect around $300 per sensor in replacement costs.

But don't throw your wallet in the street just yet. There are also plenty of little things that can make the Check Engine light come on, and many are easily corrected. Here are a few of the more commonly occurring issues:

Your gas cap isn't on tight enough.
You read that right, it might be your gas cap. Some cars measure how much pressure is building up inside your gas tank. It involves a series of mathematical algorithms that track your driving style and how much pressure is usually in the tank, then set off an alarm if it strays a certain percentage from the average. Whatever. All is means is that if you're gas cap isn't on tight, it thinks something is up and lights the orange dashboard candle, the Check Engine light. Tighten the gas cap and see what happens. It may take a week or more before the light goes out.

Your engine got wet where it didn't like it
Any electrical burp under the hood can cause one of your car's gazillion sensors to take a funny reading. When it does, you can expect to see the Check Engine light. We worked on a Ford truck once that triggered the Check Engine light every time it rained. After a lot of diagnosis, we found water that was dripping onto a spark plug wire, then running down the wire to the engine's head, causing an occasional short. Every time the water ran down the wire, the light came on. A few days later, it would turn off on its own. Be sure your engine doesn't have a wetness problem. More common than rain water getting in there is the overzealous owner who sprays his engine down at the high pressure car wash, shooting water into every crevice of the engine, thus lighting the light.

Your spark plug wires are bad
As your spark plug wires start to get old, they may develop tiny cracks which can let little bursts of electricity out. This electricity was supposed to be going to a spark plug, and since it didn't, the engine will misfire slightly, meaning one of the spark plugs didn't spark enough. Once again, this can cause the Check Engine light to come on. With your engine off, check your spark plug wires for tiny cracks or holes, especially around the ends of the wires. If they look shabby, you should replace them.