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Fault Code Reference

SPN 4094 FMI 1Aftertreatment

Aftertreatment 1 DEF Tank Temperature - Data Valid But Below Normal Operating Range

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Overview

Fault Code

SPN 4094 / FMI 1

Category

Aftertreatment

Derate Risk

No

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF tank temperature sensor is mounted directly on or inside the DEF storage tank, typically located on the vehicle frame rail or behind cab on the passenger or driver side.

Common Symptoms

  • DEF quality fault codes may also be present
  • DEF consumption inaccurate or zero reported
  • Poor NOx conversion efficiency
  • SCR system efficiency fault codes
  • DEF system disabled or limited operation
  • Frozen DEF in tank or supply lines
  • Check engine light or amber warning lamp illuminated

Root Causes

  • DEF fluid frozen or near-freezing temperature in cold ambient conditions
  • Faulty or stuck-open DEF tank heater valve not allowing coolant flow to tank
  • DEF tank temperature sensor failure or out-of-calibration reading low
  • Coolant supply to DEF tank heater circuit restricted or low flow
  • DEF tank heater lines kinked, blocked, or disconnected
  • ACM software or calibration issue causing incorrect temperature interpretation
  • Low engine coolant temperature preventing adequate DEF tank heating

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Derate Behavior

Power Derate

No

Mileage / Regen Notes

SPN 4094 FMI 1 may inhibit or delay active DPF regeneration if DEF tank temperature remains too low, as the SCR system requires DEF to be in liquid state for proper dosing. In severely cold conditions, the ACM may disable DEF dosing until tank temperature rises above the threshold, potentially leading to NOx conversion failure and SCR-related fault escalation.

System Interactions

SPN 4094 FMI 1 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

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 4094 FMI 1 — 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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