SPN 3517 FMI 31AftertreatmentInducement Risk
NOx Sensor 2 Preliminary FMI - Condition Exists
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Overview
Fault Code
SPN 3517 / FMI 31
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: NOx sensor 2 is mounted in the exhaust outlet of the SCR catalyst housing, downstream of the SCR brick, typically on the right-rear underside of the vehicle.
Common Symptoms
- NOx sensor 2 performance degradation detected
- Elevated NOx readings or erratic sensor output
- Check engine light illuminated
- SCR efficiency monitoring impaired
- DEF consumption inaccurate or excessive
Root Causes
- NOx sensor 2 (downstream/post-SCR) internal fault or degraded sensing element NOx sensor 2 heater circuit failure causing incorrect readings Exhaust gas contamination or ammonia poisoning of NOx sensor 2 NOx sensor 2 connector corrosion or poor pin contact NOx sensor 2 wiring harness damage, chafing, or open circuit Failed NOx sensor 2 due to excessive DEF dosing or SCR overloading ACM software or calibration issue causing false FMI 31 condition
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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.
System Interactions
SPN 3517 FMI 31 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 3517 FMI 31 — 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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