It's a 2001 so I would think B5 BUT the upstream O2 sensor I just replaced actually is the one listed for the 2002 model according to oreillys. I had searched oreillys and other sources before buying the plug and everything seemed to point to the Bosch part number 16307 so that's what I bought. When I actually went to swap it I found the plug end was completely different. I went to Oreillys and by comparing the plug end found that the sensor I pulled out of the car matched what was listed for the 2002 models. The store and a couple friends explained that it could be split year type thing.

While researching parts and diagnostics info I stumbled across these forums. At first glance it appears there is a ton of info on here that could be helpful and good to know but I'm not sure if I should be focusing more on B5 threads, B6, or both. As an example, I see a B6 window regulator DIY thread that might apply to me - back driver side window is being held up by rubber door stops jammed into it... a shame I know.

Does this mean most parts for my (really my wifes) 2001 A4 would be the ones listed for 2002 models or possibly a mix?