Nissan Repair: 2000 Maxima, oxygen sensor, cat converter

Question
Hello Von, my Maxima is reaching 80M miles. My Service Engine Light came on and I pulled into Auto Zone. They put it on the machine and came up with a code 0420. Explained to me it was a Cat. Converter Sensor and the part could run anywhere from $95 up to $105 depending on where it was. So, I scheduled my car into a mechanic. He advised me the 0420 code was the Converter and the part alone would be $650.00. The mechanic reset the Engine Light and advised me the problem was "borderline". If it comes back on again, I would need that part replaced.

Do you know what this code is and why the 2 different estimates?  

Answer
Tracy,

Basically the sensor is reading out of range meaning that the converter is not getting the pollution down to the level it desires.  Yes, the converter could be going out or just the oxygen sensor.  The sensor is cheaper so I would try that before buying a converter.