WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC U1000 and U1255 - Diagnostic Chart (1 Of 3)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC U1000 and U1255 - Diagnostic Chart (2 Of 3)
Courtesy of GENERAL MOTORS CORP.
Fig 3: DTC U1000 and U1255 - Diagnostic Chart (3 Of 3)