WARNING: This page is about a different car, the 2003 GMC Sierra and 2003 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC P0121: Throttle Position Sensor Performance (Steps 1-7)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC P0121: Throttle Position Sensor Performance (Steps 8-15)
Courtesy of GENERAL MOTORS CORP.
Fig 3: DTC P0121: Throttle Position Sensor Performance (Steps 16-25)