SPN 3598 FMI 3AftertreatmentInducement Risk
NOx Sensor 2 - Voltage Above Normal or Shorted High
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Overview
Fault Code
SPN 3598 / FMI 3
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: NOx Sensor 2 is mounted on the exhaust outlet of the SCR catalyst, downstream of the DEF dosing point, typically on the tailpipe side of the aftertreatment assembly.
Common Symptoms
- NOx sensor 2 high voltage fault active
- Aftertreatment system malfunction lamp illuminated
- SCR efficiency monitoring disabled
- DEF dosing may continue but without accurate feedback
- Reduced SCR conversion efficiency reported
Root Causes
- NOx sensor 2 signal wire shorted to voltage (5V or 12V supply line)
- Damaged or chafed wiring harness between ACM and NOx sensor 2 connector
- Failed NOx sensor 2 (internal short causing high voltage output)
- Corroded or water-intruded connector at NOx sensor 2 or ACM harness plug
- Open or poor ground reference circuit for NOx sensor 2
- ACM internal fault causing incorrect voltage reading on sensor input 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
SPN 3598 FMI 3 will typically inhibit active DPF regeneration as the ACM cannot accurately monitor post-SCR NOx levels. Parked and forced regens may be blocked. Derating of engine power may occur if fault is active and SCR system cannot be verified as functional.
System Interactions
SPN 3598 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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3598 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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