SPN 3226 FMI 19Medium
Outlet NOx Sensor - Data Erratic
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3226 / FMI 19
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: Outlet NOx sensor is mounted in the exhaust pipe downstream of the SCR catalyst, typically on the tailpipe or SCR outlet cone near the rear of the aftertreatment assembly.
Common Symptoms
- Erratic or fluctuating outlet NOx readings on scan tool that don't correlate with engine load or SCR performance. MIL illuminated. Possible intermittent fault codes setting and clearing. SCR efficiency codes may also be present due to unreliable NOx data. Driver may notice no drivability issues initially. Fuel economy calculations and DEF dosing may be affected due to unreliable feedback signal.
Root Causes
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Derate Behavior
Mileage / Regen Notes
This fault typically does not directly inhibit active regeneration. However, erratic outlet NOx data can cause the ECM/ACM to misinterpret SCR efficiency, which may indirectly affect DEF dosing strategy during regen. If the system cannot verify proper NOx conversion, some OEMs may limit regen frequency or duration. Monitor for related SCR efficiency codes which can inhibit regen on some platforms.
System Interactions
SPN 3226 FMI 19 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: No confirmed universal TSBs — verify in OEM service portal for specific manufacturer. Cummins, Detroit, and Paccar have issued NOx sensor-related bulletins covering connector sealing and harness routing. Check for updated sensor part numbers as early production sensors had higher failure rates on some platforms.
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3226 FMI 19 — 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.
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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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