Nox Sensor issues (Freightliner DD13)

Hey everyone and sorry if this question is posted in wrong area. Also sorry for the long text but looking for help with a frustrating issue that we can't seen to diagnose. Im new to the forum and seeking advice from some pros.
3 months ago had Nox sensor replaced by freightliner who said my One box filters were due a cleaning, Had DPFs baked overnight and DOC filter blown out. Truck (DD13) has been running great since. However, last week Engine light came on with the SPN 3216 FM 18 code (Inlet Nox sensor)
Had a forced Regen done and off I went. 150 miles later the same code came back again. so I ignored it for another 250 miles and then second alert saying Air Intake system problem. Had a second Regen done and noticed Inlet manifold pressure was very high but as the regen was going on this issue reduced to normal levels. Left again and within 150 miles the NOX sensor code came back for a 3rd time.
So had a coolant pressure test done and pressure max was 15lbs. it held pressure for the most part, dropping to about 14 in about 30 minutes. checked the EGR cooler pipes and they were dry so don't think the EGR is leaking. But each time we do the Regen it says the soot level is low but it says the Inlet manifold pressure is high until the Regen is complete.
So 3 Regens in a week and Nox sensor code is still there and we can't figure what the issue is.
Any advice would be greatly appreciated as Im about to bypass the who system and be done with it all.
Thanks and sorry for the ramble

 

We did the regens to get me home. Trust me I realize that was just a bandaid approach but 3 techs couldn't figure out what was causing the code to come up with a low soot level reading, so we did a regen to get me back on the road to home. The second one didn't throw a code. it just said Active fault. Air intake system failure 25% derate but it ran normally.

 

Unfortunately I didn’t get the second code but as far as noise yes there was a loud rattling sound coming from the one box when I was climbing hills under load. I assume that was back pressure in the one box due to the manifold pressure being too high. However the soot level readings on the computer said low!! That’s what’s confusing

 

we looked at the filter but its a new filter not even 3 months old

 

Yeah if I have to replace the one box Im gonna delete the whole thing. New one box is about $10K