Reduced engine power!!!!!

ok my friend has an 2003 silverado. his truck keeps going into reduced engine power and we don't know why. every place he has taken it to just blames it on his bags and sends him on his way. what do you suppose the real reason his truck is doing this?

My burban was doing that, and I had to have my throttle body replaced, they told me it was a common problem with these trucks, maybe look into that. It'll throw a code when it's doing it, I can't remember what it was called, but I'd get get the code checked first, then go from there.