Here are the possible causes of DTC P0420:
You can visually check for chafed or burned wiring harness on the downstream O2 sensor. If damaged, that wiring harness will need to be replaced
Open hood and let engine idle and listen to exhaust sound. Now stuff an old rag or towel in the tailpipe and listen to exhaust again in the engine compartment. if it increases, then the exhaust will need to be repaired.
If all the above are ok, then it has to have a new catalytic converter, followed by a computer reprogramming.
Catalytic Converters are covered by a Federal Emissions Warranty for 8yrs or 80,000 miles, which ever comes first