Check engine light

What is your question?

Good Morning, My check engine light has been coming on for about 6 months now but she has been running great. It comes on after about 5-10 min. of driving her no matter what the weather is. Having said this she has the 3.8L in her about two week ago I noticed a what feels like a miss at about 40 mph she has a miss but when I step on it she has great power no hesitation. Today when I was stoped at a light I noticed what seemed to be a miss. As I said this light has been coming on for a good 6 months or longer with no problem. Anyway what I would like to know is what sensors set off the check engine light?. I know I should bring her to a garage but being out of work I was wondering if someone could just tell me what I might check?. I collect old cars (Chevy's) no computers so I would know where to lok