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Fault Code Reference

SPN 3223 FMI 0AftertreatmentInducement Risk

Aftertreatment 1 Diesel Exhaust Fluid Tank Temperature - Data Valid But Above Normal

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Overview

Fault Code

SPN 3223 / FMI 0

Category

Aftertreatment

Derate Risk

No

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF tank temperature sensor is mounted directly on or integrated into the DEF tank assembly, typically located under the vehicle frame rail or cab, accessible from beneath the vehicle.

Common Symptoms

  • DEF quality warning light illuminated
  • Engine derate or power reduction
  • DEF system fault message on dash
  • Reduced DEF injection performance
  • Increased NOx emissions

Root Causes

  • DEF tank fluid temperature genuinely elevated due to ambient heat soak or proximity to heat sources Faulty or drifting DEF tank temperature sensor reporting falsely high values DEF tank coolant heating circuit stuck open allowing excessive heat transfer to tank Blocked or failed DEF tank cooling system (if equipped with active cooling) DEF tank insulation damaged or missing causing excessive heat absorption Wiring harness short to voltage on sensor signal circuit causing elevated reading ACM/SCR controller miscalibration or software fault logging false high temperature

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Derate Behavior

Power Derate

No

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

SPN 3223 FMI 0 (DEF tank temperature above normal) may inhibit DEF dosing to protect the DEF fluid and dosing system components from heat degradation. If DEF dosing is suspended due to high tank temperature, active DPF regeneration may still be permitted but SCR NOx reduction efficiency will be reduced. Prolonged inhibit of DEF dosing can lead to SCR deactivation and may trigger NOx efficiency faults, which can result in vehicle derate if dosing is not restored within OEM-defined timeout thresholds.

System Interactions

SPN 3223 FMI 0 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 3223 FMI 0 — 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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