AvantA6CVT

Anybody please help.i'm new to the world of Audi and am having problems.Gear selection is sound until the PRND display goes off and then i cannot select reverse it is an intermitant fault and will come and go while driving.i,ve had my car diagnosed and fault F125 has come up is that the multi function switch and if so where is it and what can i do with it once i find it? The local garage is clueless and said so long as it drives its ok wait to see if it fails!! a bit out of their depth .This is the only problem on an otherwise good car. Also would low ATF create such a problem .