@KaBoom - I sort of think that if you don't know about cars, it's not really going to help you to know why your check engine light is on. For example, it may be on for one problem. But it's possible for one problem in a car to create another problem, you know?
So it's totally possible for a mechanic to tell you that there are more problems than just the one causing the check engine light. I'd hate for you (or anyone else) to avoid getting something necessary done to their car just because they don't trust their mechanic! Just find a good shop that you can trust and stick with them if you don't plan on becoming more knowledgeable about cars.