What could cause p0420 *other* than a cat issue?
 

OK -- this is a bit weird.

2017 BRZ. Catless header, Edelbrock supercharger, tune. Been running that way for years now -- never thrown a code or anything until recently.

Nothing was changed, but now the car has started throwing p0420... occasionally accompanied by p0171 or p0141.

I've replaced both o2 sensors and checked for exhaust leaks. Nothing I can hear or feel with my hands. Header appears visually fine. Any time I've had an exhaust leak of any note before, I could hear it... and the last time I had a significant header crack it screamed under load. Nothing of the sort here.

Car runs great and pulls hard.

Any ideas? I'm struggling to think what could even cause a p0420 when it should be disabled in the tune...