Circuit/System Verification
WARNING: This page is about a different car, the 2011 GMC Cab & Chassis Sierra, 2011 Chevrolet Suburban, and 2011 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Ignition ON, observe the scan tool EBCM Battery Voltage Signal parameter. The reading should be battery voltage.