P0420 is catalyst efficiency fault. In my experience the o2 sensors won't fix it, the issue is with the cat itself. You can try running these o2 sensor spacers. I've used them on my old Audi that had a catless downpipe. Never threw a fault, and always set rediness monitors. Unless your cat is broken apart and rattling this is a cheap easy fix.
90 Degree Angled O2 Check Engine Light CEL Fix Oxygen Spacer Exhaust Extension Amazon.com: 90 Degree Angled O2 Check Engine Light CEL Fix Oxygen Spacer Exhaust Extension: Automotive
90 Degree Angled O2 Check Engine Light CEL Fix Oxygen Spacer Exhaust Extension Amazon.com: 90 Degree Angled O2 Check Engine Light CEL Fix Oxygen Spacer Exhaust Extension: Automotive