Theory Of Operation
WARNING: This page is about a different car, the 2009 Dodge Sprinter. However, it is still accessible from the selected car via links, so may be relevant.
The Transmission Control Module (TCM) requests torque reductions over the CAN bus during a transmission shift to improve shift quality. The TCM verifies that the ECM has performed the requested torque reduction by monitoring the response to the request. If the response is not within a specified tolerance, the TCM increments a counter, and if this counter reaches a threshold, the DTC sets.