P0420 - Catalytic Control Efficiency Below Threshold

Hi, hoping for some advice. My daughters 1997 SC2 popped on it's check
engine light this weekend. I went to Autozone for the code check and it
came back with P0420 Catalytic Control Efficiency Below Threshold - Bank 1.
Thoughts on what this really could be? I replaced the catalytic converter
about 4 weeks ago and the front O2 sensor about 8 weeks ago. When we bought
it 8 weeks ago, we were told the rear O2 sensor was replaced, no paperwork
though. I had autozone reset the light and so far, it hasn't come back on
with short in town driving.
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