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Fault Code Reference

SPN 3031 FMI 3AftertreatmentMedium

DEF Tank Temperature Sensor - Voltage High

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Overview

Fault Code

SPN 3031 / FMI 3

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • Check engine light illuminated. DEF temperature reading pegged high or showing impossible values (e.g., 250°F+). DEF gauge may read erratically or show dashes. Possible DEF quality fault codes stored alongside. Frozen DEF may not be detected properly in cold weather. Occasional DEF dosing issues if ECM cannot verify tank temperature.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault alone typically does not inhibit active DPF regeneration. DEF tank temperature is primarily used for cold-weather DEF thaw management and dosing calculations, not regen permissives. However, if the ECM cannot determine DEF temperature, it may limit or disable DEF dosing as a protective measure, which can lead to elevated NOx and eventually trigger SCR efficiency codes that WILL inhibit regen. Address promptly to avoid cascading faults.

System Interactions

SPN 3031 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components
TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins, Detroit, and Paccar have issued service letters related to DEF tank harness routing and connector corrosion. Check for TSBs specific to your chassis manufacturer regarding DEF tank connector sealing and harness chafe points near tank mounting brackets.

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.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 3031 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.

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

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