SPN 138 FMI 0Emissions
O2 Sensor Circuit Low Voltage Bank 1 Sensor 2
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Connector Pinout — in guided session
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Overview
Fault Code
SPN 138 / FMI 0
Category
Emissions
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Downstream O2 sensor (Bank 1 Sensor 2) is mounted in the exhaust pipe after the DPF/SCR aftertreatment system, typically within 12-18 inches of the outlet.
Common Symptoms
- Check engine light illuminated
- Increased exhaust smoke
- Poor fuel economy
- Reduced engine power
- Failed emissions test
- Active fault code SPN 138 FMI 0
- Excessive soot loading in DPF
Root Causes
- Failed or degraded downstream oxygen sensor (Bank 1 Sensor 2) producing voltage below expected threshold
- Exhaust leak upstream of sensor skewing readings and causing abnormally low voltage output
- Damaged, corroded, or open-circuit sensor wiring harness between sensor and ECM
- ECM or aftertreatment control module internal fault causing incorrect reference or signal voltage interpretation
- Sensor contamination from oil ash, coolant intrusion, or excessive soot loading affecting electrochemical output
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 138 FMI 0 will typically inhibit or abort active DPF regeneration events, as the ECM cannot confirm proper aftertreatment outlet conditions; repeated failed regens may escalate soot load faults.
System Interactions
SPN 138 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 138 FMI 0 — 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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