SPN 2631 FMI 4EGRMedium
EGR Mass Flow Rate Sensor - Voltage Low
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 2631 / FMI 4
Category
EGR
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: EGR mass flow sensor is located in the EGR cooler outlet pipe between the EGR cooler and intake manifold.
Common Symptoms
- Check engine light illuminated. MIL lamp on. Reduced engine power or torque in some cases. Poor throttle response. EGR system may default to closed position. Possible rough idle or hesitation under load. Black smoke under acceleration if EGR stuck open. Elevated NOx emissions. Scan tool shows EGR mass flow rate reading 0 or near-zero regardless of commanded EGR position.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does NOT inhibit active DPF regeneration directly. However, if the ECM cannot accurately calculate EGR flow, it may affect exhaust gas temperature modeling which could delay regen initiation or cause suboptimal regen performance. Monitor soot load levels — if regens are completing but taking longer than normal, this fault may be contributing. Parked regen should still be available. Clear fault and verify regen completes normally after repair.
System Interactions
SPN 2631 FMI 4 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. Cummins ISX and ISB platforms have documented wiring harness chafe points near EGR cooler. Detroit DD13/DD15 has known connector corrosion issues at EGR sensors. Paccar MX engines check for harness routing updates. Always search by engine serial number for applicable service letters.
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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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 2631 FMI 4 — 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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