Members found the real failure.

I was working on a Detroit DD15 with SPN 3251 FMI 0—DPF differential pressure above normal. The truck was showing excessive differential pressure, and it would have been easy to assume the DPF was plugged and sell the customer an expensive filter cleaning or replacement.

Truck Commander had me slow down and verify the pressure-sensor system before condemning the filter. I checked the differential-pressure reading with the key on and engine off, inspected the sensor and hoses, and then removed the pressure tubes.

Both tubes were packed with soot and giving the sensor a false restriction reading. I cleaned the tubes and sensor ports, verified they were open, reassembled the system, and checked the pressure reading again. The reading returned to normal.

I completed a parked regeneration, cleared the fault, and road-tested the truck. SPN 3251 FMI 0 did not return.

Truck Commander helped me prove the actual failure and kept us from throwing an expensive DPF at a truck that didn't need one. That's the kind of diagnostic support that saves the shop and the customer real time and money.

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.