On the 2003 GMC Sierra, the P0120 and P2135 are both pointing to a problem with the throttle position sensor. On this vehicle, the throttle position sensor is built into and part of the Throttle Body assembly. Carefully inspect the electrical connector and wire harness for any loose, broken or damaged wires. If no problems are found, many time a good cleaning can solve this problem. Using some throttle cleaner and a toothbrush and shop rag, try to clean as much build-up from the throttle bore, plate, and plate pivot points. If the cleaning does not solve the problem, then more commonly the Throttle Body assembly will need to be replaced. After replacement of the throttle body, there is no special programming needed. However, GM does recommend connecting a scan-tool and verifying proper throttle-opening and throttle-closing range. This is done with key on/engine off and using the accelerator pedal while the scan-tool is connected watching the TPS values.
Does your vehicle have a similar problem? Get connected to a Live Q&A session with a certified Auto Mechanic and get your answers on this problem now!