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Fault Code Reference

SPN 3364 FMI 4Medium

DEF Quality Sensor - Voltage Low

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Overview

Fault Code

SPN 3364 / FMI 4

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF quality sensor is integrated into the DEF tank header unit, mounted inside or on top of the DEF tank typically on the frame rail.

Common Symptoms

  • DEF quality warning lamp illuminated. MIL/CEL active. Possible DEF system warning message on dash display. SCR efficiency codes may also be present. In some cases, an inducement countdown timer may appear if fault remains active. No immediate driveability symptoms, but aftertreatment system performance may be reduced.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault alone typically does not inhibit active DPF regeneration. However, if the ACM cannot verify DEF quality, it may limit or disable DEF dosing, which can lead to reduced SCR efficiency and secondary faults that could eventually inhibit regen. Monitor for related NOx efficiency or dosing faults that may trigger regen lockout.

System Interactions

SPN 3364 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed TSBs — verify in OEM service portal. Check for manufacturer-specific bulletins related to DEF tank harness routing, connector sealing, and quality sensor updates. Cummins, Detroit, and Paccar have issued various DEF sensor harness-related service letters — search by SPN 3364 in OEM system.

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 3364 FMI 4 — 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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