Today i had the idea to find a problem on a buddys car, he has a P0420 code of course just recently had a new cat installed and a resonator.. We replaced the O2 sensors and problem still there.. Then it dawned on me.. "He had exaust wrk done and then the codes came." So i connected the exhaust port of my shop vac to the tail pipe.. And felt around the welds.. And what do ya kno, it was air leaking like crazy at the cat welds!
Cleared the codes.. And went for a drive... After 25 mins we declared it fixed! As before after pulling off a minute or 2 later it would flag

You have to be very careful checking your exhaust system that way. I did that once on one of my cars and sucked the spark plugs right out the exhaust and into the vacuum cleaner. Mind you that method is better than the other way I tried ... kept burning my lips on the exhaust pipe.

Roy, you got beer to clean up over here. I got a good chuckle out of that one.