Circuit/System Verification
WARNING: This page is about a different car, the 2007 Pontiac G5 Pursuit, 2007 Pontiac G5, and 2007 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
With the ignition ON and the engine OFF, observe the 5-volt Reference 1 and 2 parameters. The scan tool should display 4.3-4.7 volts for a normally operating system.