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SCR fault

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I'm an independent shop and new to the forum. I need some tech support on a 2012 Mack MRU613 MP7 non catalyzed system. Active codes SPN 5246/FMI 0, SPN 4094/FMI 18 keep returning. I have ran the DEF nozzle pressure test,flow test and measured the amount of DEF fluid and all shows to be good. I went ahead and replaced DEF dosing nozzle and pump filter, cleared codes and unit was returned to customer but returned a week later with same issue. I looked up a few things and found that it could be a faulty NOx sensor. I replaced both Nox sensors and performed a crystal sublimation test and it allowed the codes to go inactive. released unit and again it returned with the same codes. Could this be a possible SCR catalyst failure. Any help would be appreciated.  

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We have yet to see catylist failure   Chances are you have build up of  crystal at the inlet to the def cat box very common! pull it apart and look

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Pulled pipe and only found just a small....small amount of build up in the bottom of the inlet of the SCR. Nothing that I would call extreme.  I have seen it really bad before. 

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Do you have teck tool can you watch the temps the scr needs good heat to operate! Soot level good?  Good preasure on the doser? All sorts of things can cause this stuff to happen Even a turbo issue , are you in a cold climate? if the heater stops working it will cause the scr  not to work and wouldn't  tell you why we ran in to this last week! I think Mackpro has  got a good handle on this stuff He may chime in here shortly There are several guys here That may point you in the right direction  I am not a guru on this crap!

Edited by fjh

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Yes we have PTT. Last time I ran the regen all the temps looked great. But I don't remember what the soot level is right now but I will check We are in the valley area of California so no real cold climate here. Unfortunatley we are just a small company playing in the big boys playground so we don't get a lot of help and tech support from the delaership. I hope someone could give some insite on a direction to go. I know a little bit about this stuff and have had some training in it but sometimes you just get stumped. I appreciated you giving me any info you had.  

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Did you pump out the DEF fluid that was in the tank?  Poor quality DEF fluid will cause this. 

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Did not empty and refill def tank. I used the test strips and refractometer and it showed to be good. You think I should go ahead and replace anyway? Ill give anything a shot right now.

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This PDF check list is for  SPN5394/4094 faults but G/D will basically ask you to perform the same tests for a 4094 fault. Any type of contaminant that gets into a SCR will affect its performance-such as fuel. soot or coolant. Once the SCR is contaminated then it is bad. If everything checks out ok and you continue to have 4094 faults then the SCR may be the issue. Also, make sure all software is current in all ecu's.

SPN5394-4094 SCR Sys. Perf..pdf

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Thanks for the info turckster, I'll read this and hopefully I'll come up with the issue.

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