2016 Mack Pinnacle MP8 - SCR Efficiency Low SPN 4364 FMI 18

My 2016 Mack MP8 is throwing SPN 4364 FMI 18. The dash says SCR efficiency is low and I have 50 miles before a 5mph derate. I’ve already cleaned the DEF doser nozzle—it had some crystal buildup but it wasn’t fully blocked. The DEF pump seems to be building pressure (about 130 psi on the gauge), and I ran a DEF crystal test which came back at 32.5%, so the fluid quality is good. I'm worried the SCR brick itself is poisoned or the NOx sensors are lying. My outlet NOx is reading much higher than the inlet after the SCR warms up.