OBDII requires O2 sensor(s) behind the cat in order to keep an eye on its effectiveness. Some '95s, and all '96 and up cars are OBDII. Cars that have two cats (L&R) have four O2 sensors, typically labelled B1S1, B1S2, B2S1 and B2S2 - Bank One Sensor #1, Bank One Sensor #2, etc. Twin bank engines that only have one cat, like the SVX, have B1S1, B2S1 and B1S2 *OR* B2S2 (it does't really matter, it just means there's a third sensor.)

Ok...I did some checking on subaruparts.com and according to the diagram there are 3.

http://www.subaruparts.com/diag/?mod...ategory=440A-1

No.1 contains 2 O2 sensors and No.2 contains 1 (as far as I can see). Seems the price is $114, but Aredubjay mentioned they were $91. Am I looking at something different?

Yes, you are looking at the diagram for a 96. I was looking at the diagram for a 92 :D

http://www.subaruparts.com/diag/?mod...category=440-A