Circuit/System Verification: P0651
WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. 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 A/C Refrigerant Pressure-A/C High Side Pressure parameter with a scan tool. The scan tool should display 0.2-4.9 volt or 9-453 psi for a normally operating system.