Why does the P0420 code keep coming back in my Ford Fusion?

[Nicko]

Ugh, this P0420 code is the bane of my existence. It keeps coming back no matter what I do! I’ve already replaced the O2 sensor (twice), and the catalytic converter seems fine. Any other ideas on what could be causing it to pop up again? I really don’t want to keep throwing money at this thing without knowing what’s really wrong.