I have a 2003 1/2 ton Silverado that emmits a "reduced engine power" alarm. The chevy dealer installed a new throttle body and a wire harness for it, that was in December of 2013 and in April of 2014 it started to act up again.I brought it back for a warranty item and they said they could only guess at what was wrong because there were no active codes, only stored codes. The same thing they said when GM told me to bring it back to them. So far i checked the grounds, connectors, including the three connector in the computer box. What i need is some advice, the codes that it emitts are; P0120, P1515,P1516 and P2135.