o2 sensor

Bank 1 is the passenger side and if the code is for the catalyst system effeciency then it should be the downstream O2 (rear). Code P0420...

A little more info, as per Ford...

DTC
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Description
Indicates Bank 1 catalyst system efficiency is below the acceptable threshold

Possible Causes
-Use of leaded fuel
-Damaged HO2S
-Malfunctioning ECT
-High fuel pressure
-Damaged exhaust manifold
-Damaged catalytic converter
-Oil contamination
-Cylinder misfiring
-Downstream HO2S wires improperly connected
-Damaged exhaust system pipe
-Damaged muffler/tailpipe assembly
-Retarded spark timing

Diagnostic Aides
Compare HO2S upstream and downstream switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set

If you have to replace your 02 sensors make sure to get Motorcraft
02 sensors and not the Bosch worthless crap.