Replacement APP (Accelerator Pedal Position) Sensor. Also known as the TPS (Throttle Position Sensor).
For 1994+ computer controlled, drive by wire, trucks, vans, Hummer, etc with the TPS mounted on the accelerator pedal assembly. If your "service throttle soon" "STS" light is lit or trouble codes are stored in the computer, then the APP sensor is likely to blame.
Related Part #'s : 213-2820, 15751307