SPN 408 FMI 0EGR
EGR Sensor B Circuit High
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 408 / FMI 0
Category
EGR
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: EGR temperature sensor B is typically located in the EGR cooler outlet or mixer area of the intake manifold.
Common Symptoms
- Check engine light illuminated
- Excessive black smoke
- Poor engine performance
- Increased fuel consumption
- Rough idle
- Active derate condition
- EGR valve stuck closed or limited flow
Root Causes
- EGR differential pressure sensor supply voltage short to high voltage source Faulty EGR differential pressure sensor (sensor B) outputting high signal Shorted or damaged signal wire between sensor and ECM Corroded or contaminated sensor connector causing high resistance or voltage offset Plugged or restricted EGR differential pressure sensor ports or tubing causing false high reading Failed ECM internal pull-up circuit holding signal line high
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 408 FMI 0 (circuit high) may inhibit or disrupt active DPF regeneration by causing inaccurate EGR flow data; the ECM may default to a limp mode or disable EGR control, potentially leading to elevated soot accumulation and regen inhibit status.
System Interactions
SPN 408 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 408 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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