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Fault Code Reference

SPN 3241 FMI 2AftertreatmentInducement Risk

Aftertreatment 1 Outlet NOx - Data Erratic, Intermittent or Incorrect

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Overview

Fault Code

SPN 3241 / FMI 2

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: Aftertreatment outlet NOx sensor is mounted in the exhaust pipe downstream of the SCR catalyst, typically on the tailpipe or SCR outlet cone near the rear of the aftertreatment assembly.

Common Symptoms

  • Check engine light illuminated
  • Engine derate or power reduction
  • DEF consumption higher than normal
  • Aftertreatment system malfunction warning
  • Increased exhaust emissions
  • NOx sensor efficiency fault codes present

Root Causes

  • Faulty or degraded aftertreatment outlet NOx sensor (sensor poisoning, aging, or contamination) Loose, corroded, or damaged connector at the outlet NOx sensor Wiring harness damage (chafing, open circuit, or short) between sensor and ACM Exhaust leak upstream of outlet NOx sensor causing incorrect NOx readings Failed or failing SCR catalyst causing abnormal NOx conversion efficiency DEF dosing system malfunction resulting in excessive outlet NOx (clogged injector, pump fault) Low DEF quality or contaminated DEF affecting SCR efficiency ACM software or calibration issue causing erratic signal interpretation Inlet NOx sensor fault causing cross-comparison rationality failure Excessive exhaust backpressure affecting NOx sensor accuracy

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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 3241 FMI 2 may inhibit or delay active DPF regeneration if the ACM cannot confirm reliable NOx conversion efficiency. Depending on OEM and ACM logic, repeated or persistent faults may trigger derate conditions and disable active regen until the fault is resolved and the system is cleared.

System Interactions

SPN 3241 FMI 2 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.

Deep-dive diagnostic page: SPN 3241 FMI 2 — 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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