A 2012 Land Rover Aurora 2. 0T car. The vehicle can not be driven by "R" and "D" vehicles, the indicator lights flicker, the indicator light flashes, and the vehicle instrument information indicates "transmission failure". Fault diagnosis: (1) start the vehicle, confirm the fault phenomenon. (2) connect SDD, read fault information of vehicle, record fault code P0A90-77 GSMB, drive motor performance. (3) check the wiring plug connection of the B control unit. The connection is good. There is no abnormal phenomenon such as tightening the needle and fastening the connection. (4) check the relevant notifications, refresh the transmission control unit content, refresh the TCM control unit, and try to find that the fault is still the same. (5) use the calibration option of SDD maintenance function to calibrate the vehicle gear control unit. (6) access to relevant information. The fault location indicated by the fault code is part of the upper part of the TCM motor. (7) find out the replacement parts of the same vehicle and replace the malfunction after the replacement. (8) the fault GSM control unit is installed on another normal vehicle, and the fault code is the same as that of the original fault; thus the damage of the GSM control unit of the original vehicle can be judged. Troubleshooting: replace the GSM control unit. Fault summary: for the failure of control unit, it is necessary to check the relevant lines to prevent the control unit from being damaged once again due to line problems. If the replacement component is required to confirm the failure, it is necessary to confirm that the related lines and actuators have no problem in order to avoid damage to the replacement test parts. It is best to replace the suspected parts with the normal vehicle to verify the fault. Before replacement, it is necessary to confirm the same car allocation, the control unit pins are in the same arrangement, and the pin pins are not damaged to avoid damage to the normal vehicle.