You should get an O2 gauge and use it to check the O2 sensors. You can pick them up for about $20-$30 on ebay. They are also handy for tune ups to use to check for the correct mixture. You can also check the O2 sensors with a DVM if you know what to check for. A normal O2 sensor puts out 0-1 volt when it's heated up to about 700 degrees and hot enough to work, with .5 volt being the correct reading when you have the proper mixture for the front ones. The back ones will read much lower than that to indicate the cat is working. Most have just one sensor lead but if they are the 3 lead type and have heater elements make sure they aren't burned out and that they are getting 12 volts when running or they won't work.
Also you don't need to spend hundreds to replace O2 sensors, they all work the same but some have just 1 lead and others have 3-4 leads because they have a heating element. Just get the cheapest one you can find on ebay with or without heating element as needed. There are differences in the resistance rating of the heaters though, try and find one close to what you need as far as the ohms rating or it might draw too much current to heat up and overload the wiring.
