I’ve got one exception for you. A friend had a Nissan pickup (2000?) that was throwing a P0420 code when still under warranty. Nissan had a service bulletin to install a spacer on the downstream O2 sensor. Not sure if this was a recall or not.

He had the spacer installed by the dealer and all was good. No under vehicle inspection here unless the vehicle fails and you try to get a waiver. I’m guessing Nissan must have provided some documentation to verify this was a factory-approved modification, or it would fail a visual inspection.

 

Thanks for all the input, up to now I've been reading comments like "it won't be the cats, it'll be sensors" or "the cats will only fail if physically damaged or you've got another engine problem (oil leak / coolant leak / bad fuel mixture". Experiences here seem to be at odds with those beliefs.

The mechanic I took the car to suggested I try this outfit as a possible cost-saving:

Escapes La Cumbre Racing | Car & Motorcycle Exhaust Pipes

They claim to have a process for regenerating cats, that seems to involve cutting the old ones open, inserting some sort of rolled up impregnated metal material into the enclosure, then welding it back together. Sounds a bit Heath-Robinson to me, but what do I know? I was considering getting something like a Magnaflow or Paramount unit shipped out to Spain (if that's even possible), but if this regeneration idea isn't completely crackers I might give it a try.