Originally i figured my cat was faulty since I was getting lower gas mileage and smelling sulfur. Before I gutted the cat I wanted to get another opinion so i took it to my local mechanic. He ran a back pressure test on the cat and concluded that the converter was fine. Sooooo....He suggested that it was most likely running rich, map or o2 sensor most likely. WHere do I go fom here? I just installed a new o2 in exhaust mani, any suggestions/advice would be greatly appreciated, thanks HAN.

97 civic dx, intake, exhaust.