May 22, 2005Richard,
It depends on what year your truck is as far as the O2 sensor in the cat or after the cat. I do believe '97-'99's the O2 is after the cat '00-current the O2 is in the cat.
As far as the cat setup itself. The 1st set of O2 sensors are for the Air/Fuel ratio, the 1st set of cats act as sort of a pre-heater, the second set of cats clean the exhaust gases, then the second set of O2 sensors tell the ECU, PCM, or computer that the cats are functioning properly.