WARNING: This page is about a different car, the 2003 GMC Envoy XL and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC P0641: 5-Volt Reference Circuit Out Of Range (Steps 1-4)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC P0641: 5-Volt Reference Circuit Out Of Range (Steps 5-12)