Hiya
As Amar posted, I had this on my S8 shortly after I bought it - I lived with it for a few months while I investigated options. Mine would go into limp mode (red lights on the gear selector display) about every 10 starts - sometimes more regularly, other times not for days/weeks. Seemed to be worse if I had to move it around with a fw trips around the gear selector.
The F125 switch lives up on the side of the gearbox and confirms mechanically what the electronics believe is already happening - it is, frankly, a pain in the arse!
There are supposedly 2 part numbers, depending on which plug you have. Audi can source the correct one based on VIN. From memory the part is about £160.
Fitting it involves dropping the box down off its mounts in situ. One of the downpipes may also have to come off if you're unlucky. Clearance is actually worse on an S8 than an RS6 as the exhaust is routed closer to the box.
My mate did it for me while he was servicing my gearbox, which involved pulling the sump off anyway so he was going to have to move stuff out the way in any case.
Shouldn't be too horrific in cost terms, so if yours sounds similar then it should fix it.
The whole clean it up faffing about didn't appeal to me - especially if the labour on that bill was going to need repeating within the year because it stopped working again!
Adrian
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