Upstream, bank 1... - looks like that O2 sensor is the top candidate in failing...

I swapped mine a few days ago (X-Type, 2004, AWD, 2.5L), because I had (since I bought the car 3 month ago) a stubborn P1646 code, which just did not want to go away...

I learned on "britishparts.com" that the upper O2 sensor C2S51801 is used for X-Type AND X308!
That was music in my ears, as I had replaced (not long ago) both cats and all four O2 sensors on my X308.
According to the info I found, one way of checking, if a certain O2 sensor is dead or alive is to measure the resistance between the 2 (of the 4) wires, which have the same colour (both white or both black). I gathered that an O2 sensor seems to be OK, when I measure a resistance of let's say 3-11 Ohm. One of my old X308 upper O2 sensors measure alright - the other did not (it had kind of unlimited resistance) and neither did the one on bank 1 on my X-Type (that is the one closest to the master brake cylinder (well, at least on my RHD).
Thus, even though the upper X-Type O2 sensor had a slightly different Denso P/N than my old (operational) X308 O2 sensor, both looked the same, and if britishpart.com "say" that they are the same, I believe that.
Hence, I swapped the broken X-Type O2 sensor for the old X308 sensor and the error message was gone.

Obviously it was quite a fight to gat the old O2 sensor out, even though I temporarily removed all the vac-hoses and cables, which were in my way, and even though I had a 22mm O2-socket-tool. Nevertheless I ended up using a proper deep socket (22mm), which I managed to get over the old O2 sensor after cutting the cable off ( did this, because the proper O2-socket kept slipping.
Note: To get the vac-hose out of the way, which is stuck in the air-intake, use a flathead screwdriver to push/hold the black "ring" down, before pulling/wiggling carefully on the vac-pipe...


First I tried with the O2 socket, but it kept slipping.


Thus, I grabbed the big gun(s): Proper socket and cut off the wires... I needed the hinged wrench, which allowed me to apply a big lever in an inward angle, as the master brake cylinder would be in the way otherwise. Oh, and I applied plenty of proper penetrant spray first. Btw.: Using a 22mm spanner was not possible, as there is a bolt holding the cat onto the exhaust-manifold, which is in the way and would prevent any turn with a spanner.


Here are my old (broken and cut) O2 sensor and the replacement one.
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Note the damage the O2 spanner did already on the old O2 sensor. Had I tried any longer with the O2-spanner, I guess the hex would have been buggered beyond usability.