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Fault Code Reference

SPN 3364 FMI 3Medium

DEF Quality Sensor - Voltage High

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Overview

Fault Code

SPN 3364 / FMI 3

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • DEF quality warning lamp illuminated; MIL or amber engine warning light on; DEF system fault message on dash display; possible inducement warning countdown displayed; truck may show 'Poor DEF Quality' message even with fresh DEF; SCR efficiency codes may also be present; in some cases no driveability symptoms until derate initiates

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault alone typically does not inhibit active DPF regeneration, as it is an electrical circuit fault rather than an actual DEF quality concern. However, if the ECM cannot read DEF quality, it may limit DEF dosing, which can lead to reduced SCR efficiency and eventual SCR-related regen restrictions. Monitor for secondary SCR efficiency codes that could affect regen strategy.

System Interactions

SPN 3364 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins ISB/ISX platforms: check for bulletins related to DEF tank header harness chafing. Detroit DD13/DD15: check for DEF tank connector corrosion campaigns. Paccar MX: check for DEF sensor harness routing bulletins. Always search by chassis VIN for applicable campaigns.

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 3 — 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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