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Fault Code Reference

SPN 4374 FMI 0AftertreatmentHighRegen InhibitInducement Risk

DEF Dosing Valve - Performance Issue

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Overview

Fault Code

SPN 4374 / FMI 0

Category

Aftertreatment

Severity

High

Derate Risk

Yes

Regen Inhibit

Yes — active regen blocked

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF dosing valve mounts on the DEF decomposition tube upstream of the SCR catalyst, typically on exhaust pipe between DPF outlet and SCR inlet.

Common Symptoms

  • Check Engine Light (CEL) and DEF warning lamp illuminated. Poor or no DEF injection causing high NOx readings. Vehicle may enter derate or inducement mode. Reduced power and speed limiting. Ammonia smell from exhaust if valve stuck open. White smoke during dosing events if overfueling DEF. SCR inlet NOx and outlet NOx values nearly identical (no conversion). Possible DEF fluid leaking at dosing valve mounting location. Driver may report 'DEF System Fault' message on dash.

Root Causes

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Derate Behavior

Power Derate

Yes

Active Regen Inhibited

This fault prevents the DPF from completing an active regeneration cycle. DPF soot load will increase until the fault is resolved.

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

This fault INHIBITS active DPF regeneration. The ACM will block regen commands because proper DEF dosing is required to control NOx emissions during elevated exhaust temps. Soot loading will continue to increase while this fault is active. If soot load reaches critical levels, additional derates will occur. Parked regeneration may also be blocked depending on OEM strategy. Repair must be completed and fault cleared before regen can be performed. After repair, a forced regen should be commanded to verify system operation and reduce soot levels.

System Interactions

SPN 4374 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
TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins: Check for ISB/ISX aftertreatment TSBs related to DEF dosing module updates. Detroit Diesel: Check DDCSN for DD13/DD15 dosing valve service bulletins. Paccar: Check for MX-13 DEF doser TSBs. Note: Several OEMs have released updated dosing valve assemblies and ACM calibrations addressing premature dosing valve failures — always check for superseded part numbers before ordering replacement.

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 4374 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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