Circuit/System Verification
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
With a scan tool observe the Tow/Haul switch parameter in Body Controls data list. With the ignition On or engine running, operate the Tow/Haul switch several times, and observe the parameter as you operate the switch. The parameter should toggle between Active and Inactive when the switch is operated.