2001 Camry - P1135 Code

My check engine light was on, and returned a code of P1135 - Air/Fuel Ratio Sensor Malfunction. There was a hole in the flex pipe, so we changed out that one exhaust piece (down pipe, flex pipe, catalytic converter) But the code still pops up. I had to take it to the shop to get it inspected, and the guy at the shop(who I don't trust) says he wants to run diagnostics on the sensors, the circuitry, the computer, and to check for vacuum leaks. The car runs great, and I want to spend as little money as possible to fix this problem. But my car won't pass emissions inspection with the check engine light on related to a emissions sensor issue. My friend told me that it is most likely the O2 sensor after the converter, and that I should just change that out. But its like $140, and I have no idea if that is likely to be the problem or not. I don't want to give any more money than absoultely necessary to the shop guy, I only want to bring it back there to conclude my inspection. Anyone out there who knows what the most common problem would be in this situation that could help me out? I would greatly appreciate it!