SPN 3556 FMI 7 — Hydrocarbon doser performance

SPN 3556 identifies the parameter the control module is monitoring — hydrocarbon doser performance. FMI 7 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Regens fail to complete. Soot climbs.

• Coked doser injector tip
• Low fuel pressure to the doser
• Blocked doser fuel line

• Remove and inspect the doser tip for coking
• Check doser fuel supply pressure
• Run the doser output test

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.

Generally safe to finish the trip

Drivable home, but every failed regen puts you closer to a plugged filter.

• 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 Blue Collar 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.