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DD13 SPN 3251fmi 21 - Therealarw - 11-13-2025
Hi there guys..
I'm having this issue with one of my freightliners with a DD13 giving me a "DOC pressure not plausible" I've checked all the tubes for the inlet and the outlet. I checked to see if the there was voltage going to each sensor and I got 5 volts going into one pin and 2 grounds(if I unplug from each sensor) I've also run a DOC face cleaning for like 3 hours and the code still came on... I'm just not sure if it got the full cleaning... But this thing has been a pain in my a**, if I'm missing anything please let me know! Thanks
RE: DD13 SPN 3251fmi 21 - mikkhh - 11-13-2025
Just my experience. Differential pressure faults on those have been problem with the aftertreatment control unit itself. Not wiring or sensor issue. Sensor values are drifting also as they age, but even old sensors are not giving pressure not plausible. Getting more and more common as they age. Then somebody throws somekind of deman emulator or does somekind of crappy elimination of those systems, but edits aftertreatment module instead of using factory solution to deman them that involves throwing away both cans and their control unit + coding the truck so it will not cry about missing modules. After some time there is no communications on a c m so then all of that crappy deman goes to waste or they have old control unit fixed that would have cured all their problems in the first place.
How much new control unit costs? Used ones are lottery too and cost like a grand? At least in my area. Those engines are on benz trucks here, some forestry equipment and combine harvesters in agriculture. You need to find matching hardware control unit (some revision that starts with D...) and have somebody clone the software over OR have somebody fix the old one who knows what they are doing. Although engine control unit looks the same, they are not internally same and are not compatible.
So far the repairs have been succesful and and it costs like a grand to fix them, but we are in Europe and if NEW CONTROL UNIT is tolerable price just buy new one. Most guys "fixing" them are complete cowboys and will cause more harm than good. I would opt to repair them if trucks operate locally and even if repairs fail there is no huge tow bill coming.
RE: DD13 SPN 3251fmi 21 - Rawze - 11-13-2025
(11-13-2025 )Therealarw Wrote: Hi there guys..
I'm having this issue with one of my freightliners with a DD13 giving me a "DOC pressure not plausible" I've checked all the tubes for the inlet and the outlet. I checked to see if the there was voltage going to each sensor and I got 5 volts going into one pin and 2 grounds(if I unplug from each sensor) I've also run a DOC face cleaning for like 3 hours and the code still came on... I'm just not sure if it got the full cleaning... But this thing has been a pain in my a**, if I'm missing anything please let me know! Thanks
Pull the cans off it and have them professionally cleaned. doing hours worth of 'regens' on the truck does not clean them but about 2-3% at best, and is nowhere near the same as having them properly cleaned in oem-certified equipment.
Also, after they are cleaned properly, there is usually a procedure to reset the 'soot buildup ESTIMATE counters' in the ecm, so that it stops assuming they are still clogged up.
RE: DD13 SPN 3251fmi 21 - Therealarw - 11-13-2025
(11-13-2025 )mikkhh Wrote: Just my experience. Differential pressure faults on those have been problem with the aftertreatment control unit itself. Not wiring or sensor issue. Sensor values are drifting also as they age, but even old sensors are not giving pressure not plausible. Getting more and more common as they age. Then somebody throws somekind of deman emulator or does somekind of crappy elimination of those systems, but edits aftertreatment module instead of using factory solution to deman them that involves throwing away both cans and their control unit + coding the truck so it will not cry about missing modules. After some time there is no communications on a c m so then all of that crappy deman goes to waste or they have old control unit fixed that would have cured all their problems in the first place.
How much new control unit costs? Used ones are lottery too and cost like a grand? At least in my area. Those engines are on benz trucks here, some forestry equipment and combine harvesters in agriculture. You need to find matching hardware control unit (some revision that starts with D...) and have somebody clone the software over OR have somebody fix the old one who knows what they are doing. Although engine control unit looks the same, they are not internally same and are not compatible.
So far the repairs have been succesful and and it costs like a grand to fix them, but we are in Europe and if NEW CONTROL UNIT is tolerable price just buy new one. Most guys "fixing" them are complete cowboys and will cause more harm than good. I would opt to repair them if trucks operate locally and even if repairs fail there is no huge tow bill coming.
Yea man, I looked at all the sensors everything looked clean, i wouldn’t be too concerned if the truck was local and did local jobs but this one runs about 4-700 miles a day. It runs fine no loss of power it just keeps coming on, I’ve looked at everything on it. These freight liners have so many modules it could be anything
RE: DD13 SPN 3251fmi 21 - Therealarw - 11-13-2025
(11-13-2025 )Rawze Wrote: Pull the cans off it and have them professionally cleaned. doing hours worth of 'regens' on the truck does not clean them but about 2-3% at best, and is nowhere near the same as having them properly cleaned.
Also, after they are cleaned properly, there is usually a procedure to reset the 'soot buildup ESTIMATE counters' in the ecm, so that it stops assuming they are still clogged up.
That was the first thing I did before messing with the sensors and the tubes. I pulled both the cans off and had them cleaned out and then I went in and did the ash accumulation reset and yeah unfortunately it came back on a few days later. So it’s just kinda weird to me the light came back on. I know the thresholds on these sensors are very minimal so it’s really hard to tell but I did look at everything that had to do with the one box as far as pressure tubes and sensors. So I’m not entirely sure what it can be exactly
RE: DD13 SPN 3251fmi 21 - mikkhh - 11-13-2025
Keep in mind that 99% of demans are complete garbage on those DD13. "Factory" solution is meant for high sulphur diesel usage in foreign countries and it involves removing the aftertreatment catalysts (and control module) from exhaust to prevent sulphur becoming into sulphuric acid. Most likely you are not going to stumble upon somebody who does it properly. Keep that in mind.
What is the live data reading of that differential pressure sensor? if it shows ZERO (upstream and downstream of dpf) with engine running normally operating AND you already replaced sensor with brand new genuine part (since most likely the truck is more than couple years old already) and still it shows 0 and reading is not going to change it points to wiring or faulty module that is mounted directly to exhaust cans atleast on benz trucks. So far it has been faulty a c m module on more than couple of occasions for me at least. DPF regens will start and finish forcefully manually but the truck itself is not going to initiate them on its own.
RE: DD13 SPN 3251fmi 21 - Therealarw - 11-13-2025
(11-13-2025 )mikkhh Wrote: Keep in mind that 99% of demans are complete garbage on those DD13. ..
Hey there man thank you for the reply, yeah we didn’t Deman it. Everything is still under the emissions and runs the proper def and temps so nothings been changed there. I would have to get the Detroit diagnostic link program in order to check those voltages and readings, it’s kinda hard to really point where the problem is, I’m not 100% sure where the module is on this truck it might be actually within the ecm but I’m not 100% sure on that either
RE: DD13 SPN 3251fmi 21 - mikkhh - Yesterday
DD13 has separate aftertreatment control module A C M for scr and dpf , it is not integrated into engine ecu. EGR is controlled directly by engine ecu.
Is the dpf differential pressure sensor new original or has been replaced already?
Report back with live data readings and see if differential pressure sensor readings are going to change or stay at 0?
RE: DD13 SPN 3251fmi 21 - Rawze - Yesterday
(11-13-2025 )Therealarw Wrote: ...
That was the first thing I did before messing with the sensors and the tubes. I pulled both the cans off and had them cleaned out and then I went in and did the ash accumulation reset and yeah unfortunately it came back on a few days later. So it’s just kinda weird to me the light came back on. I know the thresholds on these sensors are very minimal so it’s really hard to tell but I did look at everything that had to do with the one box as far as pressure tubes and sensors. So I’m not entirely sure what it can be exactly
Also, someone needed to also go thru the EGR system and clean/replace all sensors, cleanout the piping, clean and inspect the EGR valve, etc... because it is always the engine that causes DPF problems.. not the DPF itself.
- Take pictures and post them here of the faces of any clogged aftertreatment filters, and especially the DPF it is complaining about it being clogged again, etc. - This will give you some clues about what is wrong on the engine side of things.
- NOx sensors are typically only good for about 250-3300k miles of reliable use before they start reading incorrectly as well. If those sensors are half million miles old+, then it is time to start replacing them as well.