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.

Thanks

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