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Fault Code Reference

SPN 3226 FMI 21AftertreatmentInducement Risk

NOx Sensor 1 Heater - Data Valid But Below Normal

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Overview

Fault Code

SPN 3226 / FMI 21

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: NOx Sensor 1 is mounted in the exhaust pipe upstream of the SCR catalyst, typically on the turbo-out or DPF-outlet pipe beneath or behind the cab.

Common Symptoms

  • MIL or Check Engine Light illuminated
  • NOx sensor heater slow warm-up time
  • Poor NOx sensor accuracy during cold operation
  • SCR efficiency monitoring errors
  • DEF system performance reduced
  • NOx conversion efficiency below threshold

Root Causes

  • NOx sensor 1 heater element failure or degradation (internal sensor fault)
  • Low or intermittent supply voltage to NOx sensor heater circuit
  • Poor ground connection at NOx sensor or ACM ground path
  • Corroded or damaged connector at NOx sensor harness plug
  • Wiring harness damage causing excessive resistance in heater supply circuit
  • NOx sensor contaminated or water-ingested causing heater performance degradation
  • ACM software issue misreading heater feedback signal
  • Failed or weak fuse/relay supplying heater power circuit

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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.

System Interactions

SPN 3226 FMI 21 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 3226 FMI 21 — 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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