I've been having some problems with my S6's auto-box lately where it doesn't seem to engage reverse gear when both the car and the outside temperature are cold.
No error or warning lights come on and the DIS shows the car as being in 'R' but there is no drive, and there is no "thunk" felt/heard that is normally present when the 'box engages and meets the resistance of the parking/foot brake, this seems to only affect reverse - engaging 'D' or another forward gear works perfectly.
This seems to only occur literally on first start up - if I let the car idle for a few mins or drive off in another gear then it goes away and reverse behaves normally.
I'm not seeing any sign of the clutch slipping or anything like that and it seems binary - either reverse engages normally and works perfectly or it doesn't at all, there's nothing in between.
UPDATE: Following GdD's advice I've had my specialist garage change the gearbox oil, they reported that when they did the change the level seemed to be low (it took more oil than came out of it) and that the oil hadn't drained completely which is to be expected since some of it will be sitting around in various parts of the system. Because of the fact that it will take a little time to circulate the fresh oil completely they have suggested that I see whether the situation improves over the next couple of days after I've put some miles on it. The problem recurred first thing this morning but I'll be doing a good 70+ miles today so that should hopefully be enough for it to take effect and I'll see what happens tomorrow.
UPDATE 2: I'd discussed doing another fluid change/flush with my garage and said we'd book it in in the new year when everyone was back from the holidays. Unfortunately the car decided to take matters into it's own hands last week and the gearbox spat it's dummy out, having trouble selecting gears and reporting fault codes on both 2nd and 3rd gears. It's now with the garage waiting for their transmission specialist to have a look. I think it is going to be an expensive month!