SPN 3031 FMI 4AftertreatmentMedium
DEF Tank Temperature Sensor - Voltage Low
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3031 / FMI 4
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: DEF quality sensor integrated into DEF tank module or inline on DEF supply line
Common Symptoms
- Check engine light illuminated. DEF temperature reading shows abnormally low, zero, or fixed negative value on scan tool. DEF gauge may read empty or erratic. Possible DEF quality fault codes stored due to invalid temperature input. DEF heater may run excessively or not at all due to faulty temperature feedback. Cold weather dosing strategy may be affected. Possible white smoke if dosing is impacted.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does NOT inhibit active DPF regeneration directly. However, if the ECM cannot determine accurate DEF temperature, it may limit or suspend DEF dosing as a protective measure. Suspended dosing will cause SCR inlet temps and NOx conversion efficiency to fall outside target, which can indirectly delay or inhibit regen if NOx-related derates are triggered. Parked regen is generally still available. Monitor DEF dosing command vs actual during regen to verify system is dosing properly.
System Interactions
SPN 3031 FMI 4 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 has issued service letters regarding DEF tank harness chafing near frame brackets. Detroit Diesel has addressed DD13/DD15 DEF sender connector corrosion in some service bulletins. Freightliner Cascadia models have known harness routing issues near tank. Always check OEM portal for model-year-specific 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.
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3031 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.
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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