Which O2 Sensor to go with-Error Code PO420

[Bengalswrx]

I have an '04 and recently came up with the PO420 engine code. Not sure if its the Cat or the post-cat O2 sensor. The car has no mods. Just wondering if there is a preference for O2 sensors, Bosch, NTK, Denso? I've cleared the code twice in a few days and it keeps coming back so i thought this would be first route to fix it. Any suggestions?

[Donkey]

Its the rear cat efficiency code. Denso or NTK is what I would use. Could also very well be a catalytic covertor issue.

[EvoEatr]

I have an '04 and recently came up with the PO420 engine code. Not sure if its the Cat or the post-cat O2 sensor. The car has no mods. Just wondering if there is a preference for O2 sensors, Bosch, NTK, Denso? I've cleared the code twice in a few days and it keeps coming back so i thought this would be first route to fix it. Any suggestions?

How many miles are on the vehicle? I have only seen a handful of rear o2 sensors go bad on well... any Subaru in general, what I can say from experience is 8/10 times its the cat. conv. Also most of the time the rear o2 goes bad it will throw a code for the sensor itself.

[Bengalswrx]

There are 71K miles on the car. I am hoping that the Cat would be covered by the federal emission warranty of 8yr/80K, however the dealer will only do this if they replace the o2 sensor...which they are charging way too much. Denso or NTK, might have to try tha route first and see if a new sensor does the trick....