2018 Cummins X15 Stuck in 5MPH Derate - SCR Efficiency Low

Question

My truck is stuck in a 5 mph derate and I cannot get the codes to clear even after a forced regen. The dash is showing an SCR performance issue and I am worried about being stuck at this weigh station for another night. The DEF tank is full and the fluid looks clear, but the computer keeps saying the conversion efficiency is below threshold.

garagegrease · Accepted Answer

The issue was a completely crystallized DEF Doser Valve (Part # 5317106) and a failing Outlet NOx sensor. I replaced the doser valve ($350) and the NOx sensor ($580), then performed an ‘SCR Performance Test’ and an ‘Aftertreatment History Reset’ using Cummins Insite. Once the counters were reset, the 5MPH derate cleared immediately. Make sure to torque the NOx sensor to 50Nm and don't use anti-seize on the threads as it can contaminate the tip.

Sarah Thompson · Answer

Check your DEF quality with a refractometer. Even if it looks clear, if the concentration is below 32.5% it will trigger efficiency codes every time you hit a hill.

Greg Walters · Answer

You need to check your DEF doser nozzle first. If it's crystallized with urea, it won't spray a fine mist and the SCR catalyst won't be able to reduce NOx properly, leading to that 4364 code.

Lina Brooks · Answer

Pull your Inlet and Outlet NOx sensors and check the readings using Insite or a heavy-duty scanner. If the outlet sensor is't Reacting to the doser injection, the sensor itself might be dead or the SCR brick is face-plugged.

2018 Cummins X15 Stuck in 5MPH Derate - SCR Efficiency Low

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