Might be too early but I have 500 miles on car and no 420/430 codes yet after I removed both cats and boiled them in citric acid powder solution for about 6-8 hours each...this is what I did in order:

new plugs
new IMT o rings
new air filter
new upper and lower plenum o rings
new throttle body o ring
new upstream/downstream O2 sensors

had 430 code again....then
cooked rear cat first, then front cat
put a can of BG 44K in gas tank to clean injectors

So far so good...76,000 miles in car

 

up to 1000 miles and no check engine light yet...but the wife said the battery light flickered on a few times so I bought new cables from Jagbits and will put them on when they come in--thanks Thermo for the suggestion...battery tested fine and alternator too.

 

I did the citric acid treatment to my front cat. I need to do it to the rear one, but getting it out looks like its not a lot of fun.