SPN 4331 FMI 4AftertreatmentInducement Risk
SCR Catalyst Reagent Tank 1 Temperature - Voltage Below Normal or Shorted Low
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Overview
Fault Code
SPN 4331 / FMI 4
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF (reductant) tank temperature sensor is typically mounted on or integrated into the DEF tank, located on the vehicle frame rail or under the cab step area.
Common Symptoms
- DEF system fault light illuminated
- Poor DEF consumption accuracy
- SCR efficiency monitoring issues
- Incorrect DEF temperature readings displayed
- DEF freeze protection strategy may not activate properly
Root Causes
- Shorted or damaged wiring harness between DEF tank temperature sensor and ACM (wire shorted to ground)
- Failed DEF tank temperature sensor with internal short to ground
- Corroded or water-intrusion-damaged connector at DEF tank temperature sensor
- Damaged or pinched wiring causing short to chassis ground
- Failed or faulty ACM with damaged sensor input circuit
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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
An active SPN 4331 FMI 4 fault may inhibit or restrict active DPF regeneration events, as the ACM relies on DEF tank temperature data to determine DEF system operability and may disable dosing under fault conditions, potentially leading to elevated soot loading.
System Interactions
SPN 4331 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
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.
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4331 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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