P0420 with new cat and sensor o2 sensor?

2005, check engine light tripped code P0420, research points to bad catalytic converter or o2 sensor...50/50 city/hwy MPG is 53 per tank. Thing is, I just replaced the catalytic converter and o2 sensor 2000 miles ago. Posted images from torque pro on the voltage readings for the upstream A/F sensor and o2 sensor...noticed while driving with ICE running at 55mph the O2 sensor voltage will occasionally bounce from 0.7 down to 0.1, so maybe bad electrical connection tripping the code? Also, isn't 4.0 volts for the upstream A/F sensor above nominal, as in a lean condition? A/F ratio cruising at 55 mph is around 14.65-14.7.

 

It was a Denso model 234-4623, purchased from Amazon and it was direct from Amazon not a third party. Only replaced the downstream o2, not the upstream A/F sensor.

This was DIY, super easy installation.

 

Updated to a very old thread...haven't seen P0420 since clearing codes a while last year but now see P0137 on occasion with lower MPG, average with 50/50 City highway mix is around 49 when it used to be 53...screenshot of torque pro readings during a 2-hour drive at 70 mph. AF sensor fluctuating between 3.9 to 4 volts, A/F ratio around 13.9-14, downstream 02 sensor showing either 0.1 volts or 0.8 volts...nothing in between. Thinking I might have to replace both the O2 and air fuel sensor but want opinions first since replacing the air fuel sensor will probably be a PITA