Here is what I know about the car and the problem (I bought it for $1k a few weeks ago)
Car was towed with transmission problems as the main fail point but it wouldn't start. Ground strap was dirty and caused the no start.
Trans was stuck in limp mode. Reset TCU and trans worked but after a long drive and fuel stop(shutting the car off then restarting) it got stuck in first. Reset codes after car was cool and worked great. Previous owner trashed torque converter and first/reverse went after minimal driving.
Replaced Tiptronic (DPT Code) with 50k used trans. (what a pain on stands in the driveway with a cherry picker in the way lol) Reset codes and trans works well.
Last little tidbit.... I can tell if the car is going to get stuck in first by the Brake light on the shifter. IF the brake light illuminates when I start the car it will work fine. If it doesn't then I have to force the car into limp mode by pulling the TCU. I have tried a known good TCU and the trans has been replaced. I'm at a loss how to troubleshoot and I refuse to wear out a low miles trans in limp mode. Help me out???
1999 A6 2.8 Tip
Added later, as a general rule if the car is cold, the trans will work. Park interlock is functional when the light is lit. You can pull the car into gear without the brake if the light doesn't illuminate upon start. I've tried 3 different multifunction switches all used) and they all do the same thing.