I was connecting the remote wire for my amplifier to a fuse in the panel to the left of the steering wheel. I accidentally connected it to the ecu fuse and soon as I took off in reverse my transmission started acting all whacky, plus shift shock big time anytime I shift into reverse. The dash lights up with PRN321, it won't change when I shift over to auto-stick mode. I assume the problem is with the transmission control unit. Does anyone know how to troubleshoot and locate this part? I wanna get a model # so I can find a use one if I need to. (99.5 A4 1.8T FWD)

 

Not sure how to troubleshoot other than to replace it with another one, but its location is easy - it's under the carpet on the passenger front floor area. Take off the door sill and kick panel, pull the carpet back, and you'll see a black box in a recess with a large cable going into it. Open the box, disconnect the big connector, and remove the silver box inside. That's the TCU.

If yours is shot, PM me the numbers on the front of it - I have a tiptronic TCU in my garage that's available if needed.