[jimh]

Have updated my signature and location.

I can read the codes. I haven't bothered yet this winter, and I can't remember what code(s) I had before, but I think it did sound F125 related.

I'm suspicious of the battery because I did some work on the car last saturday (O/S front upper control arms with the lovely pinch bolt), and when I was done, the car only just started due to flat battery.

I too would expect a failing switch to just get worse until it didn't work at all. I added that up with the flat battery and thought maybe it can set a F125 related code due to low Voltage during cranking.

I've had the car a couple of years now, and been shopping at A8 parts a few times, just never got around to posting here.
Jobs I've done on it:-
Oil cooler water pipe (Bufkin pipe) - fitted a new oil cooler while I was there.
Both front outer CV joints
Both front strut top mounts
Front upper control arms
O/S front lower control arm (rear)
Android stereo & reverse camera
All brake discs & pads (EBC Redstuff).
S4 steering wheel mod to use paddle shift
Cam cover gaskets
Injector seals

Annoyances still remaining:-
Sometimes gearbox error on cold start
Backlight of dash LSD panel does not work when very cold (making it harder to tell if gearbox has gone into limp mode until trying to drive forward).