An OBD II V6(4.0l) will have two "upstream" O2 sensors, one on each exhaust bank(manifold).
Then it will have one or two "downstream" O2 sensors depending on the exhaust system, single exhaust or dual exhaust, these are located after the Cat converter(s).
On a reader they are listed as
Bank 1 O2 #1, Bank 1 O2 #2(passenger side)
Bank 2 O2 #1, Bank 2 O2 #2(drivers side)
On a single exhaust system there would be no Bank 2 O2 #2
The #1 sensors are used by the computer to tune the fuel mix, lean/rich, these are the "upstream" sensors
The #2 sensor(s) are used to check if Catalytic convert(s) is working, these are the "downstream" sensors
Code P0420 means #2 downstream sensor is reading high carbon levels.
So either the O2 sensor is bad
Or cat converter is not working as well any more
Or engine is running way to rich<< this would be a long shot since #1 sensor "rich code" would be on as well.
You could swap any other #1 O2 sensor with the #2 sensor, or replace that O2 sensor, BUT............the #2 O2 sensors rarely fail, not never just rarely, these sensors get nice and clean exhaust after the cat converter, so last pretty much the life of the vehicle.
The #1 sensors often need to be replaced at 100k, but that's usually only if you have had engine trouble in the past with lean or rich running over long periods.
If you can live scan the O2 sensors you might get more info, #2 sensors shouldn't jump around like #1 sensors will, they should stay at a low number.
And it could be a #1 sensor because the #1 level and the #2 level is compared, that's how computer determines if Cat is cleaning up carbons from exhaust, but that is a long shot.