the guts of ur cat probably broke down and flew out ur exhaust, if u hollow the cat the p0420 code will still come up, so either buy a new cat, stock or highflow doesnt matter, or there is a way to trick ur o2 sensor post cat but i forget how to do that. other than that you can just keep clearing it.

 

hey for everyone i dunno if it was just my sit or a general thing but i was getting the same code when i first purchesed my 98 dakota upon inspecting the cat i seen it had been cut open and im assuming gutted all i changed is the cat and my muffler i did NOT change either sensor and no code since no problems either hope this helps at all