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.
Ignition ON, observe the scan tool Driver Shift Request parameter while toggling between the up shift and down shift with the driver manual shift switches. The reading should change between Upshift and Downshift.