SPN 520372 FMI 16

One of the few after treatment repairs that can be completed without tools. Just a laptop and knowledge. Need DDDL program and someone with knowledge. Can take up to 3 hours for process to complete.

 

That is basically telling you the def system is working to hard. There are a few monitoring parameters, but the one your interested in is nh3 larmf compensation factor. Your probably at 0.7xx.
To get that code to go away, that number needs to be at 0.925 min. Possible causes, usually nox sensors over 7500 hours. def pump slightly over speed( for freight, 900-1100 is ideal, but ive seen 1400-1700, no codes. Def lines kinked or leaking, def doser leaking. Def pressure should vary between 143-147, but spec is 145. Once you figure out what messed up, run a regen, then scr efficiency test.if you repaired it, that number will go up. Once it hits 0.925, that code will go inactive.

 

OP, you need to actually verify functionality via NOx sensors and DEF pump volume.

I recommend getting diagnostic link or find a competent tech to perform the proper tests to verify the actual failed part.

ACM is looking for DEF pump for a volume of 108 to 132 mL--not RPM speed as that is relative and only typical; not defining.

 

Bringing this post back alive.. I just had a WesternStar with DD15 engine come to my shop, derate. 13 to 15 active aftertreatment codes. Diag came up with DEF Pump overspeed and underpressure - it was cracked, Outlet nox sensor drift high and Inlet Nox sensor abnormal update rate.
So after changing those 3, the only code left was SCR closed loop. Now for the first time in my experience, I came up with 2 SCR efficiency tests back to back not going more than .80 LARMF..Even with A/C off and DEF doser quantity test pass, I had to change the doser anyway. After that, started a regen, then an efficiency test, LARMF went to .85 . I knew it would work this time, but the thing is, the second efficiency test did nothing. Learned from 2 full shifts the trick for this truck:
Regen
SCR Efficiency Test
Regen
SCR Efficiency Test
And it finally went up 0.92
Truck repaired. I read about some trucks needing regens between the tests, but I still don't understand it.
Hope it helps someone!