Originally Posted by MP4.2+6.0

http://www.boschautoparts.com/Vehicl...artFinder.aspx

I get Bosch 17182 as the rear position S1 sensors on either side.

I got the same # when I typed in a 2009 S8 onto the Rock Auto pain cots finder once you drill down to the O2 sensors. http://www.rockauto.com At $86.79, a better price than Amazon in this case.

When you pull your existing sensor, look for "182" or maybe 182/followed by some other number like "182/183" at the end of the long Bosch part number to confirm it is the exact one. From my experience w/ the Bosch catalog, it may not be, though apparently the W12 sensor cord lengths changed some over time. If not and you care about cord lengths or mismatch connector end issues, you could then track down the exact one from the # you find on you existing sensor.