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Fault Code Reference

SPN 3364 FMI 19Medium

DEF Quality - Data Erratic

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Overview

Fault Code

SPN 3364 / FMI 19

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • MIL lamp illuminated. DEF quality sensor readings fluctuating or jumping erratically on scan tool. Intermittent DEF quality fault codes. Possible ammonia smell from exhaust if system is overdosing. SCR efficiency codes may also be present. Driver may report DEF warning lamp flickering or inconsistent DEF level/quality gauge readings.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault typically does not directly inhibit active DPF regeneration. However, if the erratic data causes the system to read DEF quality as out-of-spec, DEF dosing may be suspended, which will reduce SCR efficiency and can lead to elevated NOx. Prolonged dosing suspension may eventually trigger SCR efficiency faults that CAN inhibit regen. Monitor for related SPN 3361, 3362, or SCR efficiency codes.

System Interactions

SPN 3364 FMI 19 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 quality sensor intermittent readings, DEF tank sender updates, and harness routing revisions. Cummins, Detroit, and Paccar have released service letters regarding DEF sensor connector sealing improvements.

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 19 — 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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