pacrefmechanic

I have been working on this 05 3500 exspress van for a 2 days now. It has the 6.0 with auto trans and I am recieving 3 codes for o2 sensors (P0054,P0059,P0060) all three tie into the same harness on the passenger side so I removed it from the back to test it easily. there is no resistancce issues between the sensor harness and the pcm terminals and I have checked to see if they were shorted together and and still no problem found. the previous mechanic that worked on it replaced all 4 sensors and still have these 3 code poping up. if any of you have ideas it would be grately appreciated I dont know if it could be a pcm problem or not I'm lost now.: confused: