This is a rant, not a call for any kind of suggestions or solutions. I posted it here because it may help some who encounter this code ...
<rant>
I pulled 0420 back in March at roughly 77,000 miles (noteworthy is that this is 3,000 miles before the cat is no longer covered under warranty). They programmed the PCM to expect less from the cat, which made the CEL go away. ....until now. I just got the code again, and the fix (now that it's no longer under warranty at 89,000) is to replace the cats. Big [censored] surprise. I love my car, but this kind of service philosophy/gank is quite unreal.
</rant>
And thanks to this forum, because even though this is upsetting, it's far from surprising. Someone (and I will try to dig up the post) mentioned that the PCM reflash is done only to get the cat past warranty coverage. That appears to be what happened here.
EDIT: Found the post, though there are many with the same sentiment:
Originally posted by projectSHO89:
Yeah, I can see that one....Now the customer pays to make the damned CEL go out and it will probably come back once the 8/80 runs out.
From here. 
