SPN 3227 FMI 3Aftertreatment
Aftertreatment 1 Outlet NOx Sensor Heater - Voltage Above Normal
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Overview
Fault Code
SPN 3227 / FMI 3
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Aftertreatment outlet NOx sensor is mounted in the exhaust stream downstream of the SCR catalyst, typically at the tailpipe outlet or SCR outlet cone.
Common Symptoms
- Check engine light illuminated
- NOx sensor heater circuit fault code active
- Possible engine derate if fault persists
- DEF system inefficiency warnings
- Increased emissions detected
Root Causes
- Short to voltage (B+) in the heater control circuit wiring harness Failed NOx sensor with internal heater short to supply voltage Damaged or chafed wiring between ACM and NOx sensor heater connector causing short to power Corroded or contaminated sensor connector causing elevated voltage on heater circuit Failed ACM output driver providing unregulated voltage to heater circuit Moisture intrusion into sensor connector causing leakage current and elevated voltage reading
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
An active FMI 3 fault on the outlet NOx sensor heater circuit will inhibit or interrupt SCR NOx conversion efficiency monitoring. Depending on platform and ACM calibration, this fault may trigger a derate condition, inhibit active DPF regeneration, or force a parked/stationary regeneration lockout until the fault is resolved and NOx sensor heater operation is confirmed functional.
System Interactions
SPN 3227 FMI 3 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.
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3227 FMI 3 — 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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