2002 Ford Taurus O2 sensors

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

The O2 sensors aren't the problem. The fault code is based on the readings of the sensor after the catalytic converter. That's How the Engine Computer determined there was a problem. You don't kill the messenger because you don't like the message.

The cause of this code is typically the catalytic converter. You can verify that by watching the switching rates of the front and rear sensors. The front one will switch between rich and lean about twice per second. The rear one should switch less than once per minute. When the rear one switches just as fast as the front one, it's because they're seeing the same exhaust gas composition, meaning no change is taking place in the converter.