Morning fellas, I bought a 2007 350 last year and had the engine code p0430 and p0420 throughout my whole ownership. I bought it with 106000 miles and there was 0 mechanical issues. There is now a stillen cat back exhaust and isr high flow cats on the car, as well as new downstream o2 sensors and i’m still having the engine code pop up, any advice for me? It needs inspected soon…

 

Look for O2 non-fouler fix on YT.

 

I already tried that

 

When the cats' efficiency gets to 94% it triggers the failed cat code. Hi-flows are there right out of the box.

I would never suggest cheating on emissions but a couple things to try if you've actually DONE no-foulers:
Remove the lowers, leave them plugged in and zip-tie to bracing, plug the bungs.
Or UpRev...

EDIT-
OEM O2s are highly recommended.