passing emissions

http://www.obd-codes.com/p0420

That's a puzzle, how does two cats go bad together, did you put an additive in the tank and melt the cores? was it running a misfire for the last year killing the cat.? go to parts and get a o2 bung extension for a side and it backs the rear sensor more away and fools it to thinking the core is cleaning things up, it cant get a good wiff of the passing carbons from an inoperable core. if results happen do the other side as you somehow got two

Many techs go to the sensors first and then check for exhaust or intake leaks and general condition of tune. This doesn't always result in solving issue. The manual suggests 5 different causes. Three have been mentioned already, other possibles are vapor cannister related and last one is possible ecm reflash due to age and miles. Smog techs that seem to know what they are doing many times suggest an ecm update to correct out of range parameters. Remember this if a po 430 and 420 code pops up together or one first then the other this doesn't mean their plugged, it means something is making them operate inefficiently.