severe miss will pump too much unburned fuel thru and destroy a cat. Something as simple as an exhaust leak can throw a P0420 code - but that wouldn't destroy a cat.
many people swap out perfectly good converters when they see 'catalytic converter efficiency low' or w'ever is part of the definition for P0420 - BUT, it often ins't a bad cat. vacuum leaks, dirty MAF sensors, misfires, exhaust leaks, bad wiring, bad rear O2 sensors - many things can throw that code.
on a used car with questionable background and present problems - best to start with the assumption that EVERY system needs to be examined for catch-up maintenance and possible repair. Examine and refresh basic consumable parts like plugs, plug wires and filters, most of the fluids, inspect important rubber components..., then maybe get some live data to see where any other issues may be.
Edited by 1 Lucky Texan, 31 January 2018 - 11:25 AM.