Which one is bad, front or rear? Check volvospeed.com (bay 13) for pics of the procedure. Fairly easy job unless the sensor is seized in the pipe. Get a can of PBlaster from Walmart and soak the sensor threads good and hour before you start. DOn't fool w WD40 as it's worthless. Car needs to be up on ramps. Make sure you get the correct one for your engine and use only Bosch sensors. Compair #'s to make sure they're the same. Sensors generally come in two types (1) "Direct fit" which has the complete wiring harness attached (more expensive) and (2) "Universal" fit that you will need to splice into the wiring that is already there with a splice kit that comes w the sensor.

How do you know either sensor is bad? Did you get some trouble codes?

1 and 2 - I would want to see the sensor to be sure it will fit - that is, threads, size, and connector are all compatible. Also like any transaction I would want to find out what recourse I had if something wasn't as represented.

3 - You will need to get under the car safely. The front sensor is obvious and the rear is under a cover plate or rock shield. Either sensor as well as the shield is likely to be tight from repeated heating and cooling as well as rust and dirt. You may need to get some good leverage and use rust solvents to loosen the parts.