Conditions for Setting the DTC: P0816
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The BCM detects a downshift request for 3 seconds in PARK, REVERSE, NEUTRAL, or D1 and for 600 seconds in PARK, REVERSE, NEUTRAL, D2, or D1.