P0420 is catalytic system deficiency.

You changed the downstream sensor, maybe you should try swapping it with the upstream sensor? They are probably the same part, and the PCM uses the difference in values between the two.

But I think what you really need to do is get it into the dealer for a diagnostic. They can hook up and evaluate the reported value from each sensor, both banks, and tell you for sure if it is the cat.

I have also found that there are no aftermarket CA legal cats. It has to be OEM.