The sensor in front of the CC helps control the fuel mixture and the one behind "checks" to see how well the CC did at cleaning up the exhaust. A bad "Pre cat" sensor can give a rich mixture (just been through this with ours) which isn't good for the CC and will ruin it over time. The Cat is passive in the process and doesn't cause the rich condition. If you install a new one w/o tracking down the cause of the rich condition, you'll ruin that one as well. The o2 sensor may or may not be faulty and instructions for checking them are in the 700-900FAQ. A vacuum leak will "fool" the sensor into enriching the mixture and a bad ECT sensor will fool the ECU into thinking the engine is stone cold all the time. Do you have a Check engine light on?

joey