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After treatment catalyst


Kyle 537
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What causes a SCR catalyst to go bad. I'm guessing over dozing of def fluid, or soot buildup. is there a way to clean one like they can bake the DPF filters?? had two in the fleet go out now. Ran SCR system text and every thing check out good, fluid dozing measures to spec. I replaced dozer to be sure it didn't hang open at times maybe.

Edited by Kyle Gandy
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Well hell, idk the dealer said they have seen only on bad. I hade one throwing a dozer code every thing was workin fine food def quality, all test checked out good. Replaced dozer did several regens and rest drives long ones code would clear then come back right after putting back into service. We had a new wrecked truck out back with only 1000 miles. Put that catalyst on the truck ran one regen and no problems sense. And now there is another on that seams to be bad.

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If there are no problems with the engine releasing too much NOX, like some sort of EGR system issue. Try replacing both NOX sensors and make sure the ECM & ACM software is up to date and perform a regen. If the fault can be cleared after the regen the problem is most likely fixed, but 4094 faults can be a pain to repair.

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Never heard of the SCR " mixing box" go bad. Are you doing the crystal sublimation regens? They are different type or regen that dont dose DEF and get hotter. There is a Mack service bulletin that has some tips on the 4094 code. It's SNM 13-022 if u have acess to the Trucks dealer portal through Mack. I cant seem to attach the bulletin or even make a copy of it for some reason. If the DEF doser seems to be working correctly, replaced both NOx sensors as said above.

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The newest NOx senor part numbers are 21636091 for the pre-SCR (bottom of the DPF) and 22014032 for the post -SCR (SCR box outlet) . Working on a truck now that came in with SCR derated 5MPH. Active codes SPN-4094 FMI-18 and SPN-5394 FMI-17. Did all the DEF doser test and all good. replaced both NOX sensor with newest sensor, updated software in engine and SCR ECM's and did crystal sub regen and a regular regen, all OK. I do about 1 a week of these.

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I have to put the catalyst back on. These guys knocked the ceramic filter out of the one I replaced in another truck (ig try into get more flow idk how they thought it would fix it) I'm no pro but I've learned a lot and I know they don't put something on the truck you don't need lol. but hopefully it works. Let m know how yours goes.

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  • 2 years later...

Ok I have a quick question i have checked everything above and all comes out fine the Egr cooler looked plugged and it has been replaced so has the veturi tube. my question is i have a truck throwing the 4094 code fmi 18 when you are doing a regen on it, the temps do not come up constantly but it will disable regen the dpf intake temp will come to 1050 and outlet will sit at 700 then outlet will come up to 1050 and inlet will drop to 700 both time regen stops and starts. I do not have any issues from filter already dropped out and checked cleaned everything tests out perfect still have 4094 and will have trouble starting when truck is sitting for about and hour or so 

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  • 2 years later...
  • 1 year later...
On 3/17/2014 at 2:15 PM, Mackpro said:

The newest NOx senor part numbers are 21636091 for the pre-SCR (bottom of the DPF) and 22014032 for the post -SCR (SCR box outlet) . Working on a truck now that came in with SCR derated 5MPH. Active codes SPN-4094 FMI-18 and SPN-5394 FMI-17. Did all the DEF doser test and all good. replaced both NOX sensor with newest sensor, updated software in engine and SCR ECM's and did crystal sub regen and a regular regen, all OK. I do about 1 a week of these.

Good afternoon I got this code SPN-4094 FMI-18 and SPN-5394 FMI-17  please help me 

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  • 5 weeks later...

I’m my notes, on a 2010 emissions truck during a regen the boost is 1.8 to 2 psi, however it doesn’t say with or without DRV valve ,  only the first year or two on the 2010 emissions year trucks used the DRV , they later made a kit to delete it along with a software download 

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