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Fault Code Reference

SPN 3227 FMI 2AftertreatmentInducement Risk

NOx Sensor 2 - Data Erratic, Intermittent, or Incorrect

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Overview

Fault Code

SPN 3227 / FMI 2

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 pipe downstream of the SCR catalyst, typically on the tail pipe or SCR outlet cone near the rear of the aftertreatment system.

Common Symptoms

  • Amber malfunction indicator lamp illuminated
  • Outlet NOx reading fluctuating or unstable
  • NOx conversion efficiency fault may also be present
  • Inducement warning or derate warning may appear
  • Poor DEF dosing control or excessive DEF consumption
  • Failed OBD NOx emissions test

Root Causes

  • Faulty or degraded downstream NOx sensor (sensor 2) element
  • NOx sensor connector corrosion, moisture intrusion, or damaged pins
  • NOx sensor wiring harness chafing, open circuit, or short to ground
  • NOx sensor heater circuit failure causing incorrect readings during warm-up
  • Exhaust system leaks downstream of SCR causing erratic NOx readings
  • Contaminated or failed SCR catalyst affecting NOx conversion and sensor plausibility
  • DEF quality issue or urea deposit buildup affecting SCR outlet exhaust chemistry
  • ACM/ECM software fault or calibration mismatch
  • CAN bus communication interference affecting sensor signal integrity
  • Failed NOx sensor control module (sensor-integrated)

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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 3227 FMI 2 may inhibit or interrupt active DPF regeneration if the ACM cannot confirm post-SCR NOx plausibility. Extended fault presence can trigger SCR derate and regen lockout, reducing allowable engine power.

System Interactions

SPN 3227 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 3227 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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