SPN 3237 FMI 2Aftertreatment
Aftertreatment 1 Outlet Gas Sensor Power - Data Erratic
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Overview
Fault Code
SPN 3237 / FMI 2
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Aftertreatment 1 Outlet Gas Sensor mounted on the exhaust outlet of the DPF/SCR aftertreatment assembly, downstream of the DPF and typically upstream of or at the SCR outlet.
Common Symptoms
- Check engine light illuminated
- Aftertreatment system malfunction warning
- Engine derate or power loss
- DPF regeneration issues or frequent regens
- Exhaust system diagnostic faults
- Poor fuel economy
Root Causes
- Faulty or contaminated Aftertreatment 1 Outlet Gas Sensor (NOx/temperature sensor) causing erratic signal output Damaged, chafed, or intermittent wiring harness between ACM and outlet gas sensor Corroded or pushed-back terminals at the sensor connector or ACM connector causing intermittent power/signal loss Poor sensor ground connection causing unstable reference or signal behavior Sensor connector moisture intrusion causing intermittent erratic readings Failed sensor internal circuitry producing out-of-range or fluctuating voltage ACM software or calibration issue causing misinterpretation of sensor signal Exhaust leak near sensor altering gas composition reading and producing erratic data
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 3237 FMI 2 (erratic outlet gas sensor data) will typically inhibit or abort active DPF regeneration cycles, as the ACM relies on outlet NOx or temperature sensor data to monitor regen completion and SCR inlet conditions. Regen may be inhibited until the fault is resolved and the sensor provides valid data.
System Interactions
SPN 3237 FMI 2 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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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3237 FMI 2 — 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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