SPN 3050 FMI 18 — SCR catalyst conversion efficiency below normal

SPN 3050 identifies the parameter the control module is monitoring — scr catalyst conversion efficiency below normal. FMI 18 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Inducement countdown and derate if not corrected.

• Poor quality or contaminated DEF
• Under-dosing from a plugged dosing unit
• Drifted NOx sensor or exhaust leak

• Test DEF concentration with a refractometer
• Compare inlet and outlet NOx readings at steady state
• Inspect for exhaust leaks between the sensors

Work the list in order and record the readings. Confirm the fault with live data before any part is ordered — a code names a circuit, never a failed component.

Short distance only

• Replacing the sensor named in the SPN before checking its wiring, connector and ground.
• Clearing the code and handing the truck back without reproducing the operating condition that set it.
• Reading one code out of a set instead of finding which fault occurred first.

Get a shop involved when the fault comes with a red lamp or a shutdown timer, when the checks above all pass and the code returns, or when the repair needs an aftertreatment service tool, a forced regen, or engine internals. Nothing on this page is a substitute for OEM service literature or a qualified technician on the truck.

Enter this code in Truck Commander Pro AI diagnostic support with the engine and the symptoms and you get the test sequence in order, per-truck history so repeat faults are visible, and a printable work order. Aftertreatment codes are covered in depth in the Aftertreatment Mastermind training. It is all in Truck Commander at $14.99 a month.