exhaust smell in cabin

There should never be exhaust smell in the car, that's kinda dangerous. How did you pass emissions with p0420? That should give you a check engine light

 

A bad weld would only make the exhaust really loud, the gases should not get into the cabin.

 

I don't know how exhaust can get inside the car, maybe if the mixture was too rich. The oxygen sensor is oem right?? Denso? I've bought cheap aftermarket Catalytic converters, not all of them work. You'll still get a check engine light from the cheap replacements.

 

could it be in the engine bay, and coming in through hvac?

 

The previous owners replaced it because why? Stolen? Sometimes the thieves are not very bright and make cuts in the wrong places. Maybe not even all the way through a pipe, just through the wall. Treat it like any other exhaust leak problem. With the car up on jacks spray along the exhaust with soap water while a helper blows air into the tailpipe with a leaf blower, and like for bubbles.

For whatever its worth, my son's POS RV had these issues and it was leaking at every junction where two pipes were joined with a gasket and some bolts. It was also leaking at the exhaust manifold due to multiple broken bolts there.

 

You probably either have the wrong donut inthe header connection or you reused the oem donut and it’s damaged.

Take it to a muffler shop.