SPN 3245 FMI 4AftertreatmentInducement Risk
SCR Catalyst Outlet NOx - Voltage Below Normal or Shorted Low
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Overview
Fault Code
SPN 3245 / FMI 4
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: SCR outlet NOx sensor is mounted on the tailpipe or exhaust outlet of the SCR catalyst housing, typically downstream of the DPF/SCR aftertreatment module, accessible at the rear underside of the vehicle.
Common Symptoms
- Check engine light illuminated
- Engine derate or power reduction
- DEF system fault message displayed
- Increased exhaust emissions
- SCR system efficiency fault codes
Root Causes
- Shorted signal wire to ground in the SCR outlet NOx sensor harness Failed SCR outlet NOx sensor (internal short causing low voltage output) Damaged or corroded NOx sensor connector pins causing signal short Chafed or pinched sensor harness causing signal-to-ground short Failed NOx sensor heater circuit causing sensor to report invalid low signal Water intrusion into the NOx sensor connector ACM/ECM internal fault (rare, after all wiring and sensor verified)
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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 3245 FMI 4 will typically inhibit active DPF regeneration and SCR dosing. The ACM cannot accurately determine SCR conversion efficiency without a valid outlet NOx signal, which may trigger SCR derating, DEF dosing lockout, and potential engine derate depending on OEM calibration and accumulated fault time.
System Interactions
SPN 3245 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
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3245 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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