I hate to even say P0420. We got it XMAS day 2021. I reset, and 1300 miles later the car was in a I/M ready. Since inspection month is October, here in PA, we can inspect a month 10 car, in 8,9,10. So 2nd week in August we had it inspected and passed. Sure enough in October, P0420. Then, to add insult to injury, the other car, which needed a month 10 inspection, in October, P0051.
Anyway, reset car 1, and like 200 miles later P0420. I thought oh no, here we go--my experience is when resetting, and it comes right back, then problem.
Anway, reset yet again, and now, 796 miles later, I/M is ready. I am hoping there is some other factor, that throws the 420 other than cat defective. How does it go nearly one year and not come back? How does it today as I type, go 800 miles and not come back? I think for the heck of it, I'd get new 02 sensors up and downstream on the side that's throwing the code, just to see.
Problem as I see it--our GM car takes 800+ miles for the EVAP to be ready. A shop told me they can't pass it with that not ready, even though online it seems to say 1 category can be not ready. So unlike my other car, which goes back to I/M ready in < 80 miles, we can't easily reset and pass emissions.
Again, seems so many factors can trigger that P0420, seems wasteful to replace the CAT(s) right away.
I just wanted to share my experience with that code, it went away for almost a year upon a single reset, then came back in weeks, and is gone again hopefully past the next inspection date. Good luck
p.s. on our car I need to replace the plugs haven't gotten around to it--original at 111k, so maybe that will help, although I've never seen any improvement on the other 2 cars when I replaced plugs prior to recommended....
