Question

How do you know if its the upstream, or downstream sensor throwing the code P0420

Answer 1

P0420 catalyst system low efficiency bank 1

This is catalytic convertor degrade, What you have is a downstream 02 sensor that monitor the catalytic when the downstream 02 sensor produces a signal that the ECM uses to calculate the oxygen storage of the catalytic , it knows the catlalytic is degrade, the cause of such quick degrade should be address first,