Unfortunately, the majority of recurrent Check Engine episodes eventually lead to some professional repair time. It's not an expensive dash light, but a nuisance. Common problems that trigger the light are emission control malfunctions. The emission control system is what your car uses to keep our air 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. Ignition systems are next.
Don't throw your wallet in the street just yet. There are also plenty of minor things that can make the 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.
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 tank pressure. Whatever. All this 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. 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.
Bad or low octane fuel
There are a number of cars that are very sensitive to misfires. These cars will display a Check Engine error with even the slightest hiccup in your engine. I've found that some vehicles prefer a higher octane fuel to run at optimum efficiency. They will run fine on any fuel in most cases, but tiny misfires, especially when the engine is cold, can bring the dreaded light. These can often be avoided by choosing a higher octane gasoline to run in the engine.