Just a curiosity question. Since poisoning is one of the possible reason why CAT becomes ineffective, will it always be poisoned (i.e. by coolant, silicone, phosphate in oil) before the downstream O2 ever did? Is there any possibility that if both of them are poisoned, or if the CAT is poisoned long enough to have the downstream O2 also poisoned, that you need to replace both?
I assume it is impossible for the downstream O2 to be poisoned with the cat still functioning well?