so how would an O2 sensor read a below threshold cat efficiency problem if its infront of the cat? or is there one between the two cats and one after the second?
as ypou point out , this is a two sided problem.
the rear sensor actually reports the ''low efficiency'' condition.
but the front sensor, being old and tired even thought it is not yet throwing a code as being bad, is the one controlling the flow INTO the cats, and this is where the problem lies.
history has proven:
you can replace the rear sensor and it usually does not correct the issue.
if you replace the front sensor with a subaru unit, or HIGHER quality, this will usually correct the problem. but not always.
there have been reports of owners replacing both sensors with subaru units and replacing the cats with subaru units and still not clearing the code. this rare but it still has happened.
and there is a work around for this, but i do not recommend it. better to find the cause and fix it.
but with subarus, it is very rarely the factory cats. if the car has aftermarket cats then all bets are off.
regarding the link above, if you concentrate on the posts by the original poster you will learn something. others may agree or disagree and post stories about their experience. but ''cardoc'' knows his stuff and has a lot of good info. read his posts and you will learn a lot about subarus.