My check engine light intermittently comes on, then goes out. I have had the truck into two separate repair facilities, one being the GMC dealership. The diagnostic tests register a code of P0171 - Fuel Trim System Lean, Bank 1. Not being even a weekend mechanic, I am not sure what this means, but it seems to have my mechanics stumped as well as they both have told me the truck runs well and are not sure why the light keeps coming on, then off. So far, I am into "repairs" for about $1000 and still have this problem - from a tune up to replacing gaskets. The truck runs good, although it seems like my gas mileage has fallen off a bit. Has anyone had this problem, and if so, how did you correct this? Thank you in advance.