p0420 Code is back after replacing CAT

A bad O2 sensor will trigger the code as well. I would suspect that the one on the upstream side is bad. If you want to pay to have them check the sensors with a scope, that's an option. You will still be stuck paying for labor. The unfortunate part is, you can't figure out what's bad until you replace it. Unless you pay for the tests.

The fact that the engine sounds "louder" than usual tells me you need a second opinion and a full inspection of the exhaust system on this vehicle. Also, it would be nice to hook up a scan tool which is capable of displaying "Live Data" such as O2 sensor function. IMO...after market parts...especially...CAT's.....are not my first choice when replacing parts. I would be inclined to clear this code...road test he vehicle and see how fast the code comes back. Then ...a scan tool inspection will provide data detailing what was going on just before, during, and after the computer set the code. This diagnostic procedure should lead one to conclude the root cause for this issue.

I did reset the code and it came back on within the first 25 miles. I think I'll take it to a shop and pay $100 to have them monitor live 02 data.

Sounds like a plan. If they decide to road test this vehicle, you might suggest they take the scan tool with them which will allow them to collect DATA when the event occurs.