2013 G37x. About 120k miles.

First time posting, but I've gone through a couple of Cat replacements and the P0420 code is still coming up. I've had pressure tests, exhaust pipes replaced, etc.
I seem to remember reading on here that a person fixed this issue but it didn't have anything to do with the exhaust, I believe it had something to do with the gas valve sensor (don't quote me on this), But said he had a similar issue where the code was still appearing after addressing any cat issues, pressure tests, etc.

Any guidance would be helpful!

 

Is the code present all the time? what happens when you clear it, does it stay off for awhile or come back immediately? Are there other codes present?
The shop manual give the following laundrylist of things:
Three way catalyst (manifold)
Exhaust tube
Intake air leaks
Fuel injector
Fuel injector leaks
Spark plug
Improper ignition timing

What it comes down to is the switching frequency and something is causing it to go out of spec.

Here is some good info How To Read O2 Sensor Live Data (Normal & Bad Condition) - OBD Advisor

Since you changed a lot of exhaust parts, you could look into intake path leaks or wiring to the sensors. If you have the stock intake system, check the accordian tubes for cracks.