It's an instant feeling of dread - heading down the highway and your check engine light comes on.
Don't panic yet. It's not necessarily a big-ticket fix, although it could be.
If you're lucky, it's just because your didn't screw the gas cap on tightly the last time you gassed up. However, other causes could also cost your thousands if there is major damage to your engine. In any case, you definitely want to have it checked right away by a trusted mechanic who will use an onboard diagnostics system code reader to determine the exact cause of your problem.
One common cuase is an aging, faulty oxygen sensor, which can cause gas mileage to plummet and emissions to soar. If you ignore it, it can damage your catalytic converter, which can also cause your check engine light to go on. Its job is to render carbon monoxide nontoxic. If it doesn't work, it really affects gas mileage. Replacement cost is around $2,000.
Another reason your check engine light may light up is a faulty mass airflow sensor, which determines the amount of air entering your engine so that the correct amount of fuel is added to the engine. If it's not working properly your car can increase emissions, stall and get bad gas mileage. MAF sensors can be damaged by a dirty air filter.
Finally, faulty spark plugs could be the culprit. If your car was built before 1996 they should be replaced every 25,000-30,000 miles. In later models they can last up to 100,000 miles. It's an inexpensive fix.
There are other reasons your check engine light may have come on. Your best bet is to have your mechanic diagnose the exact cause to keep your vehicle running safely and efficiently.