That code is thrown when the downstream O2 sensor signal oscillates at nearly the same frequency as the upstream sensor signal. Normally the downstream sensor oscillates more slowly. It is very, very unusual for an O2 sensor to go bad in such a way to trigger the P0420 code. Because that just doesn't happen, the manufacturers use that method to gauge catalytic converter efficiency. You need to have the cat replaced.
With the right equipment, a mechanic can display the signal voltages from both the upstream and downstream at the same time and confirm that it is or not an O2 sensor problem. You might want to have that done before the cat is replaced, but there will be a cost for the diagnosis.
Bank 1 is the bank of cylinders on the driver's side in the V8. For the I6, it's the front three cylinders.