HO2S-12 should be drivers side behind the cat ( the easier ones to get to ). #1 is the drivers side, #2 is the pass side ( hope my memory is not failing me right now on something so simple ).
I forget the wrench size, but it is screwed into a bung in the pipe. The harness should be a 4 pin square connector ( at least it is on my 01 ).
The O2 sensor will come with the pigtail and the connector on it, as long as the guy at the parts place gives you the correct one, it is a bolt in and plug in and go type of thing.
The harness that I refered to with maybe being corroded would be the one down by the O2 sensor itself. The pigtail I think is about 12 or 14" long.
If the O2 sensors have never been changed in 91K miles, it might be a good idea to do them all. But if you don't really want to play with them, trying some electrical contact cleaner might be a a good idea. O2 sensors are kind of normal maintenance items now, requiring changing before failure. ( at least that is the concensus around the forum these days ).
If the wiring itself seems good, then 1 of the possiable problems might be a corroded connector. This adds high resistance to the circuit, and will cause a voltage drop. The other possiable problem is the O2 sensor itself is out to lunch. That is what the other member posted being the cure for his same DTC code.
If you take a look at ordering one online or from a parts place, they are about 40 to 60 for a Bosch O2 sensor. Considering it is the easy one to get to, you might want to try changing that one out, and reset the PCM to see if it is the cure.
Like I said one other member this was his cure for the same DTC, but there are other options that is can be. Maybe taking apart the connector can cleaning it first is a good place to start. Clean both the male and female sides of the connector, and put them back together, see what happens.
The changing of all them is up to you, but concensus is this is a normal maintenance item that gets changed around 60K miles or so.
Good luck, post back what cures it for you.