Hardware Error Mode
WARNING: This page is about a different car, the 2006 Dodge Sprinter. However, it is still accessible from the selected car via links, so may be relevant.
When the TCM detects a major internal error the transmission will be placed in the permanent limp-in mode and cease all communication over the CAN bus. Once the TCM has entered this mode normal transmission operation will not resume until all DTC's are cleared from the TCM.