My check engine light came on and I had the code read. It came up as 0420 - Catalytic Converter not performing correctly. I was told that it could be just the O2 Sensor that comes after the converter. I had the O2 sensor in the front replaced last year. As anyone else had this problem and fixed it by replacing the O2 sensor? I'm hoping it is just the O2 sensor: $90 vs. $500.

I would certainly go that route in changing the O2 sensor first. You didn't say how many miles on the vehicle. If you have more than 80K the convertor could be a problem as well. They sometimes consolidate inside and break up causing a blockage and they will not do their intended job. If you need a convertor check online as you can find a large number of OEM and EPA compliant ones for your vehicle for under $200.