As far as I know, it is not easily accessible. I believe you have to drop the CAT to get to it, and here's a pdf for instructions:

http://rapidshare.com/files/427655773/O2_sensor_replacement.pdf

Before you do this, know that your car has 2 rear O2 sensors, and I'm not entirely sure which is bad. FCP lists replacement rear sensors at 129/piece. I'd probably get two and replace them both. If I had to guess I'd guess its bank two (cylinders 4-6) that is the problem one, but that's only from the P0160 that says its bank 2 (which is a code I didn't think the s80 had). I don't know for sure though, and its always nice not to have to do it twice! This job shouldn't take all that long (a couple of hours), but if you've never done exhaust work, plan on using a saturday for it. The rubber hangers can be annoying to a point that you just want to take a hammer to something...

Anyway, I'm hoping an S80 owner will chime in with some shortcuts, but I can't help there. I've got an 850 that has them sticking right out the bottom for easy access...

Always unplug your battery for 20-30 minutes after each attempt at fixing. The fuel trim will reset itself and you'll have to drive the car for a bit to figure out if it worked, but you'll cut down on false negatives..


Lastly, check the wires at the connection point for corrosion or rust. If there's some clean out the connections (don't use chemicals), and plug them in a few times to help clean it. Hopefully its just there and you won't have to replace the sensors..
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If you're not driving it "like its stolen," are you really driving?