My '95 FZJ had P0420 for years - it would appear every 6 mo. and is triggered by driving at moderate speeds for a certain period of time. I turned off the light (every 6 mo.), as my gas mileage was good and the truck ran fine. You can dump a huge bucket of cash into converters and oxygen sensors trying to fix this - make sure it's a real problem first.
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner