SPN 3700 FMI 4AftertreatmentInducement Risk
Outlet NOx Sensor - Voltage Below Normal or Shorted Low
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Overview
Fault Code
SPN 3700 / FMI 4
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: The outlet NOx sensor is located in the exhaust system downstream of the SCR catalyst near the tailpipe.
Common Symptoms
- MIL/CEL illuminated
- Outlet NOx sensor reading stuck at zero or very low values
- Aftertreatment system performance degradation
- DEF dosing inefficiency or over-dosing
- SCR conversion efficiency fault codes may accompany
- Regeneration cycles more frequent
Root Causes
- Outlet NOx sensor signal wire shorted to ground
- Faulty or failed outlet NOx sensor (internal short)
- Corroded or damaged sensor connector causing low voltage condition
- Chafed or damaged wiring harness causing short to ground
- Poor sensor ground connection at sensor or ACM
- Water intrusion into NOx sensor connector
- Failed ACM/DCU causing incorrect voltage reading on signal circuit
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Derate Behavior
Power Derate
Yes
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 3700 FMI 4 fault will typically inhibit or suspend active DPF regeneration and may disable closed-loop SCR DEF dosing control, as the ACM cannot accurately monitor tailpipe NOx levels. Regen inhibit and SCR derating conditions are likely.
System Interactions
SPN 3700 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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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3700 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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