Circuit/System Verification
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Start the engine. With the vehicle in Park, observe the Brake Switch parameter on the scan tool. The Brake Switch parameter should display Released with the brake pedal released. Apply the brake pedal and the Brake Switch parameter should display Applied.