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Fault Code Reference

SPN 3226 FMI 5AftertreatmentMedium

Outlet NOx Sensor - Current Below Normal

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Overview

Fault Code

SPN 3226 / FMI 5

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: Exhaust pipe upstream of SCR catalyst, typically 12–18" after turbo outlet on ISX15; after DPF outlet on DD15

Common Symptoms

  • No or erratic outlet NOx reading on scan tool; outlet NOx sensor reads 0 ppm or displays dashes; MIL lamp illuminated; possible DEF dosing issues due to invalid feedback; intermittent fault that clears and returns; may see related inlet NOx or SCR efficiency codes

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

Active regeneration is typically NOT inhibited by this fault alone, as regen is primarily controlled by DPF soot load and exhaust temps. However, DEF dosing strategy may be affected since outlet NOx is used for SCR efficiency feedback. If SCR efficiency faults develop as a result of invalid outlet NOx data, active regen may become limited. Monitor for secondary codes.

System Interactions

SPN 3226 FMI 5 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
TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins has issued service letters regarding NOx sensor harness chafing on certain ISX15 and ISB applications. Detroit Diesel has bulletins on NOx sensor connector corrosion for DD13/DD15. Paccar has addressed harness routing concerns on MX engines. Always check OEM QuickServe, DTNA ServiceConnect, or equivalent for model-specific TSBs.

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 3226 FMI 5 — 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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