Transmission Control Unit or Output Sensor?

Hi Guys

I have a 2006 Audi A6 2.0Tdi Auto/Multi 130k with full service history. A few days ago i noticed the PRNDS on the screen were all in block red (all lit up). The car goes into all the gears perfectly fine but it dont show which gear im in as they are all lit up on the dash. The only thing that is effected would be on take off the car is very sloppy as if its taking off from 2nd gear. This problem sometimes goes away at some startups and then comes back while driving and other times its there on startup. When the problem is not there then i can tell the car power is normal at take off.

I have also noticed since the above problem that my mpg has dropped dramatically to 12-15mpg!!!

I took it to my local reputable garage and he put the Snap-On computer and shows this:
"18201 Transmission output sensor 2 G196 No signal P1793"

The guy said this means its the gear box speed sensor thats nackered. He called the dealer and gave the dealer the code and they mentioned that the codes displayed do not match with the actual problem ( i dont know what they mean by that). They said its best to bring it in to them for a detailed diagnostics, first of all i know they are really expensive and 2nd of all the waiting list for a diagnostic check in 4 weeks! This is the same for all 4 dealers within my locality.

I would like to know weather the Transmissin control unit is gone or is it actaully the output sensor thats showing up on the snap on?

Many many thanks for your kinds replies.