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Fault Code Reference

SPN 3031 FMI 19Medium

DEF Temp Sensor - Data Erratic

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Overview

Fault Code

SPN 3031 / FMI 19

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • DEF temperature reading jumps erratically on scan tool data, DEF temp shows impossible values (spikes or drops), MIL/CEL illuminated, DEF quality warnings on dash, possible DEF dosing issues due to incorrect temperature calculation, frozen or boiling DEF false readings, intermittent fault codes logging in history

Root Causes

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Derate Behavior

Mileage / Regen Notes

Active regeneration is typically NOT inhibited by this fault alone, as DEF temp is used primarily for dosing calculations rather than regen control. However, if erratic data causes the system to believe DEF is frozen or overheated, dosing may be suspended which can lead to elevated NOx and eventual SCR efficiency faults that WILL inhibit regen. Monitor for secondary SCR efficiency codes.

System Interactions

SPN 3031 FMI 19 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, Detroit, and Paccar have issued bulletins on DEF tank harness routing and connector sealing; search by SPN 3031 in OEM system. Check for TSBs related to DEF tank sending unit assembly updates specific to your engine platform.

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 3031 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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