O2 Sensor or Cats?

[One Red Ride]

Not on the Mustang, but the generic codes should be the same for my '97 Taurus. I have a P0430: Catalyst System Efficiency Below Threshold (Bank2) MIL that keeps coming and going. Does that mean an O2 sensor on the Bank2 side is bad or the cat is plugged/bad? The car has 145,000 miles, so I'm think'n the later, but I'm not sure.

Any other problems this could be that I'm not thinking of? Sorry for the off topic , but this group seems to be the most knowledgable out there ...