emissions inspection blues

[towncarblue]

i have a 2002 nissan pathfinder 4x4.
ive been having the p0420 p0430 codes. i replaced both of my post cat O2 sensors and the pre cat O2 sensor on the driver's side . but the codes kept coming back. i tried to do the spark plug fouler trick , it cleared out the p0430 code [ code never popped up again ] but the p0420 code keeps coming back on.
i went and cleared the code , drove the pathfinder for about an hour and went to the emissions inspection station. they could not read enough info from the obd2 so they did a rear muffler front fan test.
i failed, High NOx Readings .
two days later i noticed that the pipe between the rear mufflers had completely rotted out. my mechanic went and fixed the rotted pipe and told me to replace the mufflers because they are rotting out. after he repaired it i noticed the pathfinder drove a bit better.
now im not sure what to do ? should i replace the passenger side catalytic converter ? could a broken rear pipe cause the p0420 code ? could the broken pipe cause me to fail inspection?
i dont know what exhaust part to start buying first?

[Towncivilian]

I don't think an exhaust leak after the downstream O2 sensor would cause a problem unless it's very close to the sensor, but I'd get it fixed regardless. Check for vacuum leaks and clean the top end using something like Mopar Combustion Chamber Cleaner or Amsoil Power Foam, and run a couple tanks with Chevron Techron (20oz bottle) to improve the chance of passing emissions after you've fixed the codes.

Since you've tried replacing the downstream O2 sensors and it didn't resolve the codes, I would replace the cats. Even though your spark plug non-fouler trick worked, your cats still may be partially clogged or worse, which would lead to poor performance and worsened fuel economy. Check RockAuto for reasonably priced catalytic converters, and be sure to use a 5% off discount code if you decide to order anything from there.

[towncarblue]

it turned out that the resonator on the p0420 side was clogged/broken/restricted .. the exhaust shop replaced the resonator with straigt pipe