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Fault Code Reference

SPN 3227 FMI 16AftertreatmentInducement Risk

NOx Sensor 2 Preliminary FMI - Data Valid But Above Normal

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Overview

Fault Code

SPN 3227 / FMI 16

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: NOx Sensor 2 is mounted in the exhaust pipe downstream of the SCR catalyst, typically on the tailpipe or SCR outlet cone, near the rear undercarriage of the vehicle.

Common Symptoms

  • Check engine light illuminated
  • NOx sensor out-of-range fault code active
  • Elevated NOx readings displayed on diagnostic scan tool
  • Possible engine derate depending on emissions control strategy
  • SCR system efficiency monitoring affected

Root Causes

  • Faulty or degraded downstream NOx sensor (Sensor 2) reporting elevated NOx values SCR catalyst degradation or failure causing insufficient NOx conversion DEF (Diesel Exhaust Fluid) quality issue — diluted, contaminated, or incorrect concentration DEF dosing system malfunction — clogged injector, failed DEF pump, or restricted DEF lines DEF doser injector carbonization or crystallization causing under-dosing Exhaust system leak upstream of SCR catalyst reducing reductant effectiveness NOx sensor 2 connector corrosion or wiring harness damage causing signal offset SCR catalyst poisoning from lube oil ash or fuel sulfur Incorrect DPF or SCR catalyst installation or wrong catalyst for application ACM calibration or software issue causing improper DEF dosing command

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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 16 can inhibit or suspend active DPF regeneration if the ACM determines SCR system performance is out of range. Depending on OEM and fault severity, the system may enter a derate condition limiting vehicle speed or engine power. DEF dosing may be suspended by the ACM pending diagnosis. Parked or forced regen may be required after repairs to clear aftertreatment system status flags.

System Interactions

SPN 3227 FMI 16 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 16 — 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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