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Fault Code Reference

SPN 3228 FMI 2AftertreatmentInducement Risk

Aftertreatment 1 Outlet NOx Sensor - Data Erratic, Intermittent or Incorrect

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Overview

Fault Code

SPN 3228 / FMI 2

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: Outlet NOx sensor is mounted in the exhaust pipe or aftertreatment housing downstream of the SCR catalyst, typically near the tail pipe exit.

Common Symptoms

  • NOx sensor reading erratic or fluctuating beyond normal range
  • Check engine light illuminated
  • Aftertreatment system efficiency faults
  • DEF consumption calculation errors
  • SCR conversion efficiency monitoring issues
  • Intermittent fault code active/inactive cycling

Root Causes

  • Faulty or contaminated outlet NOx sensor element
  • Exhaust leak upstream of outlet NOx sensor causing false readings
  • Corroded or damaged NOx sensor connector/harness
  • NOx sensor heater circuit intermittent fault causing slow heat-up and erratic data
  • SCR catalyst efficiency degraded causing unusual NOx readings that trigger rationality fault
  • DEF dosing system malfunction causing skewed NOx conversion readings
  • ECM/ACM software issue causing misinterpretation of NOx sensor signal
  • NOx sensor poisoned by improper DEF fluid (high urea concentration or contaminants)
  • Wiring harness chafing or intermittent open/short on NOx sensor signal circuit
  • Failed or failing SCR catalyst leading to outlet NOx values outside expected range

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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 3228 FMI 2 may inhibit or delay active DPF regeneration if the ACM cannot confirm post-SCR NOx levels are within expected range. Repeated faults may trigger a derate or regen lockout depending on OEM strategy.

System Interactions

SPN 3228 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 3228 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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