1999 Mazda Miata P0420 Code

I purchased the car from a dealer on 07/28/09. Prior to purchasing, I spoke with the previous owner and asked if he had recently replaced the catalytic converter. He said yes that he bought an OEM from Advance Auto Parts and it cost him $595.00. He did not offer any additional information. I used my OBD-II reader to look for trouble codes or pending codes and there were none. After purchasing the car (on the 140 mile drive home) the check engine light came on. My OBD-II reader gave the P0420 code. I have turned it off twice but each time it has come back on. Any thoughts as to if something else could be causing a rich condition and throwing this code?

I suspected the Advance part was not up to OEM standards. What do you mean by "are all the monitors set?"

I am using an Autoxray EZ-Read 2000. After I scan the data and press the "Mon" key, it tells me the following:
MIL Status ON
Misfire Completed
Fuel System Completed
Comp Component Completed
Catalyst Completed
Heat Catalyst Not Supported
Evaproative System Completed
2nd Air System Not Supported
A/C Refrig Not Supported
O2 Sensor Completed
O2 Sensor Heater Completed
EGR System Completed

Is that what I should have been looking for? When I select Capture Mode, O2Tests, Test Results, or Vehicle Identification under the Mon menu, I am prompted to contact Autoxray for an update. When I read the trouble codes under data is when I got the P0420 code; and there are no codes when I look for the pending codes. I appreciate your willingness to help me out and your knowledge about such matters.