SPN 3364 FMI 2 — DEF quality poor

SPN 3364 identifies the parameter the control module is monitoring — def quality poor. FMI 2 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Countdown to a 5 MPH derate over the next few hundred miles.

• Water or diesel contaminated in the DEF tank
• Failed DEF quality sensor
• Cheap or expired DEF

• Test DEF with a refractometer (32.5% urea)
• Drain and refill with fresh DEF if off-spec
• Check the DEF tank header sensor

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

You have a limited mileage window before a 5 MPH derate. Head straight home or to a shop now, do not add another leg.

• 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.