1996 Toyota Camry Air Conditioning Malfunction

Greetings Bob, Ken, and 2carpros team! I have a 1996 Toyota Camry Sedan- L4-132 2164cc 2.2L DOHC(5S-FE) To begin the catalytic converter under the car has been changed twice. It's not a monitored catalytic converter, should it be? My check engine light is on and the code came up as PO420. I took it to a different mechanic and he said the converter under the hood needed to be changed and it was. I'm picking up the car in the morning. As far as my Air conditioner is concerned, my old mechanic put in a new aftermarket compressor and a new freeze dryer and filled it with r134a, It's still not working. What do you think is causing the A.C. Not to work? Also, What is the difference between a monitored catalytic converter under the vehicle and a non-monitored one? What are the risks involved in leaving the non-monitored one in there? I know I have asked a few questions, but I really need your proffesional advice on these issues. I also forgot to mention that the new compressor gets really hot after just a few minutes. I Thank You in advance for answering my questions in depth. You guys are great over there!