SPN 3700 FMI 3AftertreatmentInducement Risk
Outlet NOx Sensor - Voltage Above Normal or Shorted High
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Overview
Fault Code
SPN 3700 / FMI 3
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: Outlet NOx sensor is mounted in the exhaust stream downstream of the SCR catalyst, typically on the tailpipe or SCR outlet cone near the rear of the aftertreatment assembly.
Common Symptoms
- MIL or Check Engine Light illuminated
- SCR efficiency fault codes present
- Inaccurate NOx readings or NOx values stuck high
- DEF dosing may be affected or inactive
- Reduced engine performance or power loss
Root Causes
- Outlet NOx sensor signal wire shorted to voltage (B+)
- Damaged or chafed wiring harness between ACM and outlet NOx sensor
- Failed outlet NOx sensor (internal short)
- Corroded or water-intruded sensor connector causing high voltage on signal circuit
- ACM connector pin damage or spread pin causing cross-circuit voltage
- Incorrect sensor installed (wrong part number causing signal mismatch)
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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
FMI 3 on the outlet NOx sensor will typically inhibit active DPF regeneration and may disable SCR dosing control; the ACM cannot accurately monitor tailpipe NOx levels, triggering regen lockout and possible derating on many platforms.
System Interactions
SPN 3700 FMI 3 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 3700 FMI 3 — 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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