Circuit Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 Chevrolet Tahoe, 2002 Chevrolet Suburban, and 2002 Cadillac Escalade EXT. However, it is still accessible from the selected car via links, so may be relevant.
The Vehicle Communication Unit (VCU) and the Vehicle Interface Unit (VIU) communicate using the OnStar® Serial Data (-) circuit, OnStar® Serial Data (+) circuit and the OnStar® Serial Data Return circuit. DTC U1500 is set when the VIU detects no communication between the VIU and the VCU.