Isb scr conversion efficiency nightmare

[dslmchnc]

I've got 2 isb Cummins in freightliners that have the same conversion efficiency and regent quaility codes. During the scr test the catalyst is the only thing that fails. We replaced one and it still has the same issue. I believe the inlet nix sensor is causing our issues. What should I be seeing for ppm during a Regen? And the inlet should be higher than the outlet correct?
Thanks