No signal from A/F sensor and persistent P0420

2001 3.4L V6 with aftermarket cat back exhaust, K&N air intake, and two new after market cats.

I'm trying to fix a persistent P0420 ODB code. I've replaced both stock catalytic converters (this is a california model) as well as my stock exhaust (cat back), spark plugs, wires, pcv valve, cleaned the MAF, and replaced the A/F sensor with a Denso A/F sensor. My next guess is to replace the downstream O2 sensor but I don't want to do that until I figure out why I have not signal from my A/F sensor.

My guess is it is either the EFI relay, which the A/F sensor passes through or it's the harness the A/F sensor signal passes through. Does anyone else have experience or insight into this problem? I'm getting my ODB information through Torque and it shows no signal from the A/F sensor whatsoever however the engine does go into closed loop when it is warm which I believe is an indicator that the A/F is working.