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Fault Code Reference

SPN 401 FMI 0EmissionsMedium

Exhaust Gas Recirculation Flow Insufficient Detected

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Overview

Fault Code

SPN 401 / FMI 0

Category

Emissions

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: EGR valve and cooler are typically mounted on the intake or exhaust side of the engine block, near the intake manifold, accessible from the top or side of the engine.

Common Symptoms

  • MIL or CEL illuminated on dash
  • Reduced engine power or derate active
  • Excessive black or gray exhaust smoke
  • Rough idle or unstable engine operation
  • Failed emissions test or high NOx readings
  • DPF regeneration frequency increased

Root Causes

  • EGR valve stuck closed or severely carboned up restricting flow
  • EGR cooler clogged or failed reducing exhaust gas volume through circuit
  • EGR differential pressure sensor failed or sensing lines blocked
  • EGR piping or inlet tube heavily carboned or collapsed restricting flow path
  • EGR actuator or solenoid electrical failure preventing valve operation

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Derate Behavior

Mileage / Regen Notes

Insufficient EGR flow causes elevated combustion temperatures and increased PM/NOx output, which can increase DPF loading rate, trigger more frequent active regens, and may inhibit regen if engine derate is active.

System Interactions

SPN 401 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.

Deep-dive diagnostic page: SPN 401 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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