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Thinking about RS6 C5 (4B) upper O2 sensor (S1) replacement. Luckily the ones on my car were done less than 20K miles ago so I suspect they will last for a while. But, in light of some testimonials I have come across it just strikes me as odd that a whole engine drop is required for this service.

Yes, I have read about owners who just dropped the subframe 4-5 inches and were able to reach both O2 sensors that way, but I also came across a post a few years ago by a Danish owner who explains how the Shop he relied on built a special tool to reach the upper sensors without need to dislodge the engine at all. Does that make sense to any of you who have done this on your own in the past?

Obviously, that would be the Holy Grail solution. Logistically, building such a long-reach tool shouldn't be unthinkable: what's your opinion? Do you think there is potentially enough clearance to reach the upper O2 sensors with the engine in situ using a purpose-made tool?

I've heard of people doing this, but I have always seen them use these types of tools in addition to dropping the engine 4-5 inches via the subframe. It would make less sense to try to do it in situ in comparison to how relatively easy it is to lift the car off the subframe to give you the extra bit of space. I saw a guy on YouTube replace his turbos this way with a special made tool where he welded the XZN bit to the end of a flexible hose clamp driver. Might have been the same guy you're referring to. His video doesn't really have much step-by-step DIY other than just showing it can be done.