MazdaSpeed248

My engine check light is being set off by my rear O2 sensor i know this because my car was diagnosed by MAZDA. Mazda tried fixing this the first time i brought it in because it read that the front sensor was dead and needed to be replaced so they replaced it and the car's light turned off for like 2 days then it came back. I went back and they told me it was the rear one because it was Bosch not the same brand as the OEM one in the front. So i like WHAT and decided to get some answers myself went to pep boys to give my car a smoke test because lately you can smell the car's gas like its not burning well or running rich and yes that what it was no leaks just the engine running rich due to the rear o2 sensor not working properly because i took the catalytic converter out. So am left with two options O2 Simulator Mini Cat Cel Fix best oxygen sensor simulator made. or tuning the car which i'd rather not do please help does that mini cat o2 sensor extender work PLEASE HELP!