Hugh,

I just did the rear sensor today - it was a pretty easy job.

I bought the O2 sensor tool made by Lisle from a local auto shop and followed the instructions on Bay 13 (http://volvospeed.com/maintence.htm). It took some torque to loosen the sensor, but it wasn't that bad.

I'm cheap, so I bought a generic O2 sensor (Bosch number 15717) for about $50 and spliced it into the existing wiring. I think this is actually easier than buying the "proper" sensor, because it's a whole lot quicker to cut and splice wires by the sensor than to thread a long lead through the whole engine bay. All Bosch 4 wire sensors are basically the same - the difference is in the connector and wire length (oh, and price!). The 15717 is for various Fords.

So for about $50 and about 1.5 hours labor, I replaced one sensor - the other would have been an additional 30 minutes, but that had been replaced fairly recently by the previous owner, so I left that one alone.

For a scan tool, go to scantool.net and buy theirs for about $50. This is one you plug into a laptop (assuming you have a laptop, of course) and it works very well. It will read and reset codes and read various sensors. It uses open source software, so that is free. My 850 didn't have a check engine light, but the scan tool was showing an O2 fault, which is now gone. Now to see if my gas mileage gets where it should.