SPN 4331 FMI 5AftertreatmentInducement Risk
SCR Catalyst Inlet Temperature - Current Below Normal or Open Circuit
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Overview
Fault Code
SPN 4331 / FMI 5
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: SCR catalyst inlet temperature sensor is mounted on the inlet cone or inlet pipe of the SCR catalyst housing, typically on the aftertreatment assembly under or behind the cab.
Common Symptoms
- Check engine light illuminated
- Engine derate or reduced power
- DEF dosing disabled or reduced
- Exhaust aftertreatment system malfunction warning
- Poor SCR conversion efficiency
- Increased NOx emissions
Root Causes
- Open circuit in SCR catalyst inlet temperature sensor wiring harness Corroded or damaged sensor connector causing high resistance or open Failed SCR inlet temperature sensor (thermistor open internally) Chafed or broken wire between sensor and ACM connector Pushed-back or spread terminal in harness-side connector Poor pin-to-socket contact at ACM module connector Water intrusion into connector causing corrosion and open circuit ACM internal driver fault (rare)
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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
FMI 5 (open circuit) on SCR inlet temperature sensor will typically inhibit active DPF regeneration and SCR dosing strategy optimization. The ACM cannot confirm safe exhaust temperature conditions for regen or verify SCR catalyst operating temperature, resulting in regen inhibit or abort. On most platforms, a warning lamp will illuminate and a derate or regen lockout may occur until the fault is resolved.
System Interactions
SPN 4331 FMI 5 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4331 FMI 5 — 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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