2003 Chevy Silverado Check Engine Light/Sluggish Performanc

Check engine light came on. I took it to our local Chevy dealership. Initial quote was $280. Later looking at the quote, they were pricing replacing the spark plugs and making a steering column adjustment. They reset light but could not find the problem. They thought it might be a loose gas cap. I thought "check engine" is either related to ignition or emission problem? Plugs were replaced at 100K miles (along with the other required servicing). Check engine light has come back on, and I don't feel comfortable bringing it back down to the dealership. Believe they're too interested in just selling service. Truck seems to be running okay, but I have noticed a loss in power. Acceleration is slow when I jump on it. We live in a small town and I do not have a mechanic I can trust. Any suggestions for possible repair would be appreciated. Thank You!