Q: My car shows trouble codes P0133 and P0420

hello. I had my car serviced about 2 months ago the car's ob was throwing off a p133, p420 and p171.. I took it to the shop, the oxgen sensor was replaced. .I Drove the car for a couple of days and the ob was throwing off just p420, I was told that if i add catalytic converter cleaner to the fuel tank its going to clean the cataltic converter..so i bought the cleaner but a couple days later still get p420. Am thinking ok i might need a new catallytic converter, but since it's expensive i wait until a month later to try to have it replaced..before i have it replaced i check the ob with handheld scanner now am getting a p133 and p420. I should not be getting a p133 since i had the oxgen sensor replaced about a month ago. when i take the car back to the mechanic that replaced the oxgen sensor he tells me maybe i should replaced the catalytic converter first, that the p133 code might go away. i think he is jsut giving me the run-around..can u tell me what to do?? thanks

My car has 182000 miles.
My car has an automatic transmission.