More rear O2 sensor help

[brus brother]

OK I'm thoroughly confused.

I have P0420 code - for bank 1 (passenger side in US) catalytic efficiency below threshold

1.) MY 05 Outback has a total of 5 sensors. Since my 05 Outback has one precat and one postcat sensor and then another on the rear cat after the passenger and drivers pipes meet, do I need to change all three at 130,000 miles and while I'm at it throw in the drivers side precat and postcat sensor just because??

2.) Are any of these critical OEM?

Dealer told me I needed a new cat (maybe 2). YIKES x 2!

 

Edited November 2, 2011 by brus brother