The oxygen sensor is the most likely cause of the check engine light. Even if you do not drive the car too hard, the sensor is a general maintenance item at about every 50K miles or so. The oxygen sensor looks similar to a spark plug, and is on the exhaust manifold or pipe. It is a very simple repair. Purchase a new (Not used!!!) sensor (about a $100 for my car), remove the old one w/ a wrench, and screw in the new one. The hardest aspect is to trace the wire from the old sensor and plug in the new wire at the same location. Voila! You are done. The best place to buy a sensor is The Benz Bin. They are by far the cheapest that I have found.