po420 means the rear o2 is reading very similar or the same as the front o2 in a 2 o2 system. on a 4 o2 system its just the one bank. id say its the cat...ive done all the diag on a 01 in the last week, even did mopars recomend if the downstream is new and upstream is old replace it, code came back...so most likely its the cat, and while you have the cat out replace the 02s, you need to take them out anyway to weld in the cat or replace the headpipe. I would only run a mopar 02 the aftermarket parts are crap. also an aftermarket cat will meet the spec provided by the manufacturer, it will be on the bottom end of the scale tho. usually aftermarket cats dont last nearly as long as an oe cat, they dont use the same amounts of precious metals...which is why if you recycle an aftermarket cat you get 10 bucks an oe cat you get 75.