Yeah, if there's not actually an obstruction the best way to get rid of the p0420 is just a spacer. Mine it wasn't super obvious that it was crumbling. It would still get to highway speeds but it was slower than normal. The main indicator I got of obstructed flow was I would be accelerating in second and it would get up around 5500 and just kinda hangout there instead of climbing fast and "banging" The rev limit. It would get to the limiter but it was noticeably slow from 5500 to 6200 and the car actually pulled harder if you shifted into third which is not normal at all for a car in good shape but makes sense as a clogged cat increases backpressure too much and lower rpms it affects it less. My fuel trims were high but not throwing codes except maybe once in a blue moon and only when it got so bad I could hardly keep it at 83mph on flat ground low altitude floored in 4th.