DTC U1500: ONSTAR® (Generation 2.6) Serial Data Circuit Malfunction: Description
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
Vehicle Communication Unit (VCU) and Vehicle Interface Unit (VIU) communicate using OnStar® Serial Data (-) circuit, OnStar® Serial Data (+) circuit and OnStar® serial data return circuit. When DTC U1500 is stored, VIU does not communicate with VCU, no phone calls can be placed, and OnStar® status LED turns Red. A history DTC clears after 50 consecutive ignition cycles, or VIU receives a clear DTC command from scan tool.