Circuit/System Verification
WARNING: This page is about a different car, the 2008 GMC Yukon XL, 2008 GMC Yukon, 2008 Chevrolet Tahoe, 2008 Chevrolet Suburban, and 2008 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
Start the engine in PARK and observe the Ignition Voltage parameter on the scan tool. The Ignition Voltage should be between 12-14.5 volts.