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I'm wondering if your question has to do with how to change it, or where to find it.

If the former, it's relatively easy if you're patient. First, of course, you need the special socket (it has a groove to slip over the wires), which you can get from IPD (about $20 or so). Then, presoak with frequent applications of PB Blaster over several days. And here's a tip -- I found it easiest if you first try to loosen it when the pipes are hot! Sure, it's harder on you (be careful), but I believe that the pipe and the hole expand to loosen the grip on the sensor's threads -- a cold pipe is a contracted (tighter) pipe, IMHO.

If the latter, then yeah, it surprised me, too, in discovering that it's by the cat. My older 240's always had it conveniently on the exhaust manifold -- then I discover that the latest (last) 240's have it tucked way under the middle of the car (hard to get to, even on ramps). Bummer!