Circuit/System Verification
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet 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. Operate the Tow/Haul switch several times, and observe the parameter as you operate the switch with the transmission in DRIVE. The parameter should toggle between ACTIVE and INACTIVE when the switch is operated.