Replacing the downstream O2 sensors will NEVER fix a 420/430. Failed sensors will store other codes. Possible the upstream sensor is lazy or contaminated causing chemical shift, but the most likely cause is a failing cat. If the cat isnt broken up and causing excessive back pressure, the easy fix are some "MIL Eliminators" sold by many online stores including Amazon and Ebay for around $20 a pair. Simply remove the downstream sensor, screw in the device, and screw the sensor into the device. They work.