SPN 4364 FMI 18?

[Unilevers]

this means that the nox reduction from the aftertreatment inlet and outlet nox sensors is not high enough. in otherwords the scr isnt working. if you drive it long enough with this issue it will get steadily increasing derates until your down to 1000RPM.

Most of the time it is a bad nox sensor. it can also be a bad scr injector, bad urea, urea crystals in the decomp tube, wrecked SCR, bad dosing pump/plugged filter, or a leak in the urea system.

force a regen and compare inlet to outlet nox - when the scr is dosing the outlet nox should be much lower than the inlet nox (outlet should run about 25ppm).