i'm with O.
suabru cats will last the life of the car if you do not damage them by driving with a blinking CEL. of course they can go bad, but they usually do not. but some dealers and some shops love selling new cats when the owner gets a P0420.
replace the front o2 sensor with a subaru or HIGH quality o2 sensor. (cheap sensors may not fix the problem.) you want the front one working really well since it controls the air fuel mixture.
the rear o2 sensor just checks to see if the cats are doing their job. and almost any sensor will work in this position.
but the cats rarely fail in subarus. buying aftermarket cats is a waste of money.
some owners have had their emissions check even when they had a p0420 code, and they passed the test. this means that on some suabrus, even when they have a p0420 code the exhaust emissions are with in federal specs.
do not replace the cats.
it is not the cats.
replace the front o2.
there are lots of things that can cause a p0420 code. the front o2 sensor is the most common. but there are others.
check this out: