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Fault Code Reference

SPN 3372 FMI 0AftertreatmentInducement Risk

Aftertreatment 1 Outlet NOx 1 - Data Valid But Above Normal

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Overview

Fault Code

SPN 3372 / FMI 0

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: Outlet NOx sensor (Sensor 1) is mounted on the aftertreatment outlet pipe, downstream of the SCR catalyst, typically near the tail pipe exit or on the outlet housing of the aftertreatment assembly.

Common Symptoms

  • Check engine light illuminated
  • Engine derate or power reduction
  • Elevated DEF consumption
  • Poor fuel economy
  • Aftertreatment system fault warnings on dash

Root Causes

  • DEF (Diesel Exhaust Fluid) quality issue — diluted, contaminated, or incorrect concentration (below 32.5% urea) DEF doser/injector clogged, stuck open, or malfunctioning — insufficient or erratic urea spray pattern SCR catalyst degraded, poisoned, or thermally damaged — reduced NOx conversion efficiency DEF supply pump failure or low DEF delivery pressure — insufficient urea reaching SCR Outlet NOx sensor fouled, failed, or drifted high — false high NOx reading Inlet NOx sensor failure causing incorrect reductant dosing calculation DEF tank heater failure in cold climates causing frozen or gelled DEF Exhaust leaks upstream of SCR catalyst — reducing catalyst inlet temperature and DEF mixing DPF regeneration system malfunction causing abnormal exhaust temperatures affecting SCR performance ACM software/calibration issue resulting in incorrect dosing strategy

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

SPN 3372 FMI 0 (outlet NOx above normal) will typically inhibit active DPF regeneration as the aftertreatment control module interprets elevated outlet NOx as a system protection condition. Depending on OEM threshold logic, the system may enter a derate, disable regen, and increment SCR inducement counters leading to progressive engine derates if not resolved. Continued high NOx events may trigger SCR inducement shutdown sequences.

System Interactions

SPN 3372 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 3372 FMI 0 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.

Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.

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