Code P0420?

I have a 2000 Ford Ranger with a 3.0 V6, my check engine light came on and it is throwing the code P0420 manifold catalyst efficiency low bank 1. I replaced the downstream oxygen sensor and cleared the code, drove the truck for one day and the check engine light came back on throwing the same code. What is the problem? Is the catalytic convertor bad? Could it be the mass air flow sensor? Even though the check engine light is on the truck seems to be running fine, I just want to fix whatever is causing the engine light to come on.