A common problem with wiring harness on SMG3 cars is the wiring harness for gear position sensor located on top of transmission. The wiring is made of biodegradable insulation and is prone to deterioration over time. If harness is not moved insulation might stay in place but over time the vibration will make insulation flake off exposing copper conductor.
Once copper core is exposed it is just a matter of time before the different pinouts come in contact with each other. This causes erratic feedback values to go back to SMG DME confusing the processer and triggering numerous codes or erroneous drivability issues. There is no set code for this problem because it is a feedback sensor and the various shorts will lead DME to assume that another part in system is causing transmission to not respond.
It is recommended to either change whole harness or re-sleeve original wires with new insulation.
MLRENG: Repair harness PN: 30.110