My B6 S4 auto has developed a transmission problem that puts it into limp mode after a couple of miles of stop and go driving. The car seems to drive fine until this occurs, though in limp mode the shifts are rough (is limp mode always like this?)

I took it to the dealer for diagnostic and the problem seems to be the transmission control module. Unfortunately, that's a ridiculously expensive ($2600) part on these cars. I called an independent garage and they quoted me the same price, as the only place you can buy this part is directly from Audi (I had a friend who works for an auto parts supplier look into this too).

My service manager advised that I could consider replacing the transmission itself. Of course the dealer wouldn't install a used part, but I could buy a used auto trans for under $2000 and the labor at an independent shop is probably in the $1000 range. So that would actually be cheaper than just replacing the control module.

The diagnostic report reads"76178 RAN GFF FOUND FAULTS P0706 TRANSMISSION RANGE SENSOR A CIRCUIT RANGE PERFORMANCE AND P1754 TIPTRONIC SWITCH UP OPEN CIRCUIT / SHORT CIRCUIT TO B+ RECOMMEND TO REPLACE TRANSMISSION CUSTOMER DECLINE REPAIRS AT THIS TIME"

Has anyone with a tip B6 S4 had an issue like this? The service manager at the dealer said they hadn't seen a TCM issue on a B6 S4 before, usually it's just the CVTs with this problem.

Honestly, at this point I just want to get the car fixed and sell it.