Action Taken When the DTC Sets

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.

The TCM illuminates the malfunction indicator lamp (MIL).
When failure occurs in a forward range and shifting is complete, the transmission remains in the current range. When failure occurs while in a forward range and the shift is in process, the transmission returns to the previous range, except in post-shift state, where the transmission will continue to the commanded range. When failure occurs in other conditions, the transmission shifts to 1st, 3rd or 5th. While diagnostic response is active, if the shift selector is moved to NEUTRAL or REVERSE, or any other forward range, the transmission will lock in NEUTRAL.
DTC P0721 is stored in TCM history.
The TCM freezes shift adapts.
The TCM inhibits torque converter clutch (TCC) engagement.