Circuit/System Verification
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
With the scan tool installed, ignition ON and the engine running, observe the Battery Positive Voltage parameter in the TBCM data list. The reading should be between 9-17 volts.