P0420 Diagnostic and Fix for 1999 Honda CRV: O2 Sensor Replacement Not Enough?

I'm experiencing a recurring P0420 diagnostic code in my 1999 Honda CRV. After clearing the code, it returns within approximately 20 miles of driving. I recently replaced the oxygen sensor (which I believe is the only O2 sensor on this model), but the issue persists. Using an IR thermometer, I've measured temperature differences before and after the catalytic converter—no significant drop, which suggests the converter isn't malfunctioning. Given that the P0420 code indicates a failure in the exhaust system's post-catalyst efficiency, I'm wondering if there are other potential causes beyond the O2 sensor. Any insights or troubleshooting steps for this specific 1999 Honda CRV model would be greatly appreciated.