You have 4 O2 sensors. One before and one after each of your two cats. THey are about $50- $60 EACH. There is a testing proceedure for them with a voltmeter, (not that I know what it is)
I just replaced one of mine, I bought a less expensive O2 sensor socket to remove it, because a better brand was not locally available. It started to round the corners of teh sensor. Fortunately I was able to get a 3/4 inch box wrench on it after I cut the wires and connector off. The connector would not fit through the 3/4 in box. So once I cut the wires I was committed to getting it out. FOrtunatley it worked for me. I would recommend that one purchase a better QUALITY O2 sensor socket if you can't get at it with a wrench. A regular deep socket will not reach even if you cut the wires off right at the sensor. | 
