Also while doing it, O2 sensors are OE Bosch. I expect if you track down the Bosch #, it will also be cheaper than the Audi dealer sourced one. All the forward (S1) sensors are accessible from under hood by the way, so if for any reason you decide to change more of them than just the problematic S2 one, you really don't need to worry about later access to any of the front four.

I'm running BG44K fuel system cleaner every 5K as a preventative maintenance. It does not burn and since I don't have FSI, cleans the intake system and O2 sensors. Hopefully I don't have to worry about the O2 sensors. Fingers crossed.

HOLD THE PHONE ON O.P.!
 

FWIW, I never had issues w rear sensors. Only front.

But wait... For OP, are you sure it is the sensor itself? Are you getting a faulty reading or codes telling you that, or are you getting electrical type codes for sensor? Especially a code saying either short to ground or no signal or something like that. If the latter, big percentage chance it is just the connector on the car side of harness. I had at least two harness connectors go bad on my 2006. Earlier I changed some front sensors (not hard) but code came back.

A known W12 issue and there is a TSB on it. Fix is the replace car side connector. The wiring terminals inside the connector have some bonding issue and go bad. All the connectors on a W12 are near the (US) passenger side air box. Rear sensor connectors are a different part number than fronts (fewer terminals since they are not heated like fronts) They also have black and brown (color coded by side) ones, depending on which it is. I can explain further if your situation is such it may be connector. Like 100x easier if it is this wiring related issue than anything dealing with those couple high up S2 sensors.

Thanks MP4! :) The fault is for the rear sensor in bank 2(Driver side). It says that there is NO signal so the idea about a bad connector can definitly be it. As you mentioned it I remember that I also had 2 front sensors that gave up errorcodes but seam to work fine now(after a visit to the mechanic), so I guess the problem could be electric. As you say, 100x more easy! I will need to have that checked out!

I did search for Bosch sensors too and found a very nice (read: cheap) german supplier at qp24.de that I will order from.

See my reply 13 to this post. https://www.audiworld.com/forums/a8-...p2254-2868359/ That gives you some "how to" and part #s for the terminal/conductors you need. The connector part # is for the S1 (forward) type, but you can should be able to get the rear ones from the dealer (or a parts listing to reference) too. If no signal, I would cross fingers and bet it is simply the connector issue. Then no need to do anything with sensors in place and just some harness related minor work.

Sometimes you can at least improve them by unplugging and replugging in connectors and spraying with some terminal type cleaner. In my experience only temporary though. Issue is apparently how the wire terminals were crimped onto the individual wires. The wires (other than the heat ones on front sensors) are also very small and I suspect may internally break after some routine handling and movement with accumulated motor related work--as simple as servicing the air box on that side since they are mounted to it and often need to be moved aside.

Help me - what am I looking at? Passenger side wheel well?

Yes, with liner removed of course.

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Did you cut the floor
 

what did you end up doing ?