First, repair any exhaust leaks.  They can confuse the computer into
thinking there's a catalyst issue.

Then, to decide whether the oxygen sensors are working properly, you'll
need to look at their waveforms.  The front one should cycle from low to
high voltage when hot idling at least once every three seconds.  The rear
should remain relatively steady.  If the amplitude of the rear oxygen
sensor waveform is similar to that of the front oxygen sensor waveform,
then you need a new catalytic converter (the one between the two sensors).