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Fault Code Reference

SPN 3226 FMI 6AftertreatmentMedium

Outlet NOx Sensor - Current Above Normal

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Overview

Fault Code

SPN 3226 / FMI 6

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

  • Erratic or abnormally high outlet NOx sensor readings on scan tool. MIL lamp illuminated. Possible DEF dosing issues due to faulty NOx feedback. Intermittent fault codes related to SCR efficiency. Dashboard warning for aftertreatment system. NOx sensor data may spike or read maximum value. Possible ammonia smell from exhaust if DEF is being over-dosed based on bad sensor signal.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault typically does not inhibit active DPF regeneration. However, the faulty NOx signal may cause the ACM to miscalculate SCR efficiency and DEF dosing, which can indirectly affect overall aftertreatment performance. If combined with SCR efficiency codes, the system may limit regen frequency or request parked regen. Monitor for secondary codes that could trigger regen inhibit.

System Interactions

SPN 3226 FMI 6 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 TSBs — verify in OEM service portal. Cummins ISX/ISB platforms have had service letters regarding NOx sensor harness routing near exhaust. Detroit DD13/DD15 has had bulletins on aftertreatment harness chafing. Paccar MX engines have had connector seal updates. Always check OEM service information for current TSBs by VIN.

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 6 — 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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