Getting P0420 DTC(Catalyst System Efficiency Below Threshold (Bank 1) Took a look with amazon dongle, seems like bank 1 sensor fluctuating quite a bit. See image.

Is this a bad sensor, or would bad cat also read the same?

 

Upstream O2 sensors are supposed to fluctuate anywhere from .1-.9 mv the down stream when at operating temperature and cruising at a steady speed should be in the .650-.750 range and not fluctuate to much. You can rent a tool to do a back pressure test. Also you can use a temp gun to see if it is restricted or not operating correctly anymore. Remember catalytic converters are heaters it should get hotter from inlet to outlet.