The 420 code is for a failed catalytic converter. The precious metals are worn out inside and the gases are not being cleaned.
It is never the O2 sensors as they are doing the job of monitoring the catalytic converter.
The way to confirm is to have a scan tool and monitor the voltages of the downstream O2 sensor. It should have a steady reading. If it varies like the front, then the catalytic converter is bad.
Catalyst Efficiency (Bank 1)
Aging or Contaminated Oxygen Sensor (O2S)
Engine Mechanical Condition
Exhaust Gas Leak
Oxygen Sensor (O2S) Wiring and Connectors
Saturday, November 17th, 2018 AT 11:20 AM