P0420 is a generic OBD code indicating an problem with the catalytic converter efficiency of the bank 1 catalytic converter on a Honda Accord. This does not necessarily mean that the catalytic converter is bad. Some other parts should be inspected further:
Spark plugs
Ignition wires
Fuel injector
PCV valve
Vacuum lines
Air intake filter
If all these parts are still good and the code is still present then it is likely a bad catalytic converter.
