p0430 cat efficiency, Help

I know by having the computer scanned that this code is pass side cat efficiency issue.. I hear its most likely the o2 sensor, possibly the cat itself. What are the chances its the cat..
If it wasnt for the cost factor Id just replace all 4 o2s and be done with it. Im also worried it may not be the O2 and be the cat itself. I heard to test the o2 sensors you get the vechicle up to op temp, and check milivoltage on the sensor wires and look for fluctuations between.02 and .08 I believe? There are 4 wires coming from the O2 which 2 of the 4 do I probe...... Also do I disconnect the sensor and probe in the harness or should I pierce the wire while its connected. Im hitting this project tomarrow morning along with a water pump so quick replys are appreciated. Its a 98 Tahoe 5.7. Thanks Tony
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