2001 Dakota code P0420

Okay, so a friend of mine has a 2001 Dakota, single cab, 2wd, V6. He just bought this thing and it started missfiring and pulled a code. He did the worst thing possible and let advance auto pull his codes and tell him what was wrong with it. The code was the downstream O2 sensor. He replaced that. A week later, it starts missfiring and pulling and upstream O2 sensor fault. He replaced that. Now he brings it to me. The code he is now pulling is a P0420 (Catalyst system efficiency below threshold bank 1). He has already replaced the air filter, plugs, wires, etc. My guess was a partially stopped up cat. Flow meter shows that the cat. is good. I cannot find anything wrong with the truck. Freeze frame, fuel trim, flow data all look good. I don't know what to tell him at this point, and he is taking up one of my lifts! The truck does have after market exhaust on it. I checked for TSB's but there are none for that system on this truck. Any ideas?

would low grade gas have anything to do with it??

Keep us posted as to what you find. I know a guy on another forum with the exact same problem with his 03 v6 Dak, can't seem to fix the problem either!!

make sure the cat is not gutted and make sure who
ever modified the exhaust didn't relocate the downstream o2 sensor did they install dual exhaust from the manifolds or from behind the rear o2 if it's from the manifolds then the rear o2 is not getting enough exhaust gasses so it wont be hot enough to operate properly and also if he installed an aftermarket o2 that could be the problem also

If the downstream sensor went first I would say he has a cat problem. could be it's been running rich for a while and fried the cat.

its DEFINITELY a cat problem. but there is also another problem that caused the cat to go bad. they do go bad over time but thats after a lot of mileage. as long as the engine is running good and not rich then replacing the cat alone should cure the problem. sounds like hes done everything else already.