Found the real failure without throwing parts at the truck
I was working on a Cummins with Fault Code 3582—SPN 4364 FMI 18, SCR conversion efficiency below normal. This can be a complicated fault because the problem could be contaminated DEF, an exhaust leak, bad NOx-sensor data, insufficient DEF dosing, or a damaged SCR catalyst.
Truck Commander walked me through the diagnostic process instead of automatically condemning expensive aftertreatment parts. I checked the exhaust system for leaks, verified the DEF quality, reviewed the inlet and outlet NOx readings, and performed a DEF dosing test.
The dosing quantity was below specification. I removed the DEF doser and found it heavily restricted with crystallized DEF. The doser and decomposition area were cleaned, the dosing flow and spray pattern were retested, and the system passed.
After completing the repair, I ran the SCR system test, cleared the fault, and verified that Fault Code 3582 did not return. Truck Commander helped me find the actual failure without wasting money throwing NOx sensors or an SCR catalyst at the truck.