O2 Sensor Data

[3rdGENrunner]

Ok So I was told that to determine which one of my catalytic converters is blown that I could hook up a scanner to my O2 sensor. I told him the OBD II code was P0420 (which just says that one/both isn't working). Is this guy bogus? Could I just rent the scanner from Autozone and do it myself? If so how do I read it?

*Please help!*

The mechanic told me it would be $100 to do this