how do i see if O2 sensor needs fixing?

It is located in the exhaust system, the only thing with wires.. If you have a check engine light that says it is bad then replace it. If not don't bother. Again get a muffler shop to check the converter. they have a machine that will tell you if it is bad.

what if i have had problems before were they thought that the 02 sensors were wrong but they never checked it. would it be wise to replace and see if it is the wiring and not the actual converter before attempting to replace the converter. or does the OBD test give 100 percent accuracy that the problem is the converter and not the 02 sensor

Nothing in the automotive industry is 100%. OBD 11 scanners are as accurate as the computer reads out. The best test for a ctalytic converter is a device that is put in the tail pipe and it measures back pressure and emissions at the end of the exhaust system. The OBD 11 scanner only says that the catalytic converter is not efficient.

The efficiecy is directly linked to engine effiiency including all the computer controls that make the engine run in specs. Changing the O2 sensor is shooting in the dark. An OBD 11 scanner can monitor the voltage swing of the O2 and a mechanic can tell if it is working properly.

Good Luck