The O2 cell after the cat tells the pcm that the cat is working that is it's only job. The up stream O2 cell is responsible to tell the pcm if rich or lean that is key! The magnum swap I did was from a 1996 truck. It had 1 up steam O2 cell and 1 down stream cell. I used a H pipe for crossover and in the H pipe I welded a bung and that was were the non foul adapter and down stream O2 cell was located. My engine was a little fat at idle due to camshaft change of duration and overlap, other than that it ran fine. I did not have any codes.
