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Fault Code Reference

SPN 4374 FMI 18Medium

DEF Dosing Valve - Low Flow

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Overview

Fault Code

SPN 4374 / FMI 18

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF dosing valve is mounted on the decomposition tube or mixer assembly upstream of the SCR catalyst in the exhaust system.

Common Symptoms

  • Low DEF injection rate observed in live data. SCR inlet NOx higher than expected. SCR outlet NOx not dropping properly during dosing events. Possible white crystalline buildup (DEF residue) around doser nozzle area. Reduced NOx conversion efficiency. DEF consumption lower than normal for operating conditions. MIL lamp illuminated. Possible ammonia smell from exhaust if doser is partially clogged and spraying erratically.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault typically does not directly inhibit DPF active regeneration. However, if the SCR system cannot effectively reduce NOx due to low DEF flow, the aftertreatment control module may limit regen frequency or extend regen intervals to prevent excess NOx emissions during regen events. Parked regen may still be available. If additional SCR efficiency faults set alongside this code, active regen may become inhibited until DEF dosing is restored to normal operation.

System Interactions

SPN 4374 FMI 18 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed universal TSB numbers — verify in OEM service portal for specific make/model. Common OEM service campaigns exist for DEF doser crystallization issues. Check Cummins QuickServe, Detroit DCC, Freightliner ServiceLink, PACCAR ESA, or Volvo VCADS for applicable bulletins related to DEF doser low flow and crystallization concerns.

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.

Deep-dive diagnostic page: SPN 4374 FMI 18 — 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.

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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.

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