SPN 3237 FMI 3AftertreatmentInducement Risk
Aftertreatment 1 Outlet NOx - Voltage Above Normal
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Overview
Fault Code
SPN 3237 / FMI 3
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: Aftertreatment outlet NOx sensor is mounted on the exhaust pipe downstream of the DPF/SCR system, typically near the tailpipe outlet or SCR outlet housing.
Common Symptoms
- Check engine light illuminated
- Engine derate or power reduction
- Aftertreatment system malfunction warning
- Increased DEF consumption
- Failed emissions test or OBD readiness monitor not complete
Root Causes
- NOx sensor signal wire shorted to voltage (battery or ignition source) Faulty or failed aftertreatment outlet NOx sensor (internal short causing high voltage output) Damaged or chafed wiring harness causing signal line contact with high-voltage circuit Corroded or contaminated NOx sensor connector causing signal voltage spike Failed or failing NOx sensor control module/heater circuit affecting signal output ACM/ECM internal fault misreading NOx sensor signal voltage
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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 3237 FMI 3 fault will typically inhibit active DPF regeneration as the ACM cannot confirm proper aftertreatment outlet NOx levels; parked and active regens may be blocked until the fault is resolved.
System Interactions
SPN 3237 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.
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OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3237 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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