SPN 2631 FMI 2Medium
EGR Mass Flow Rate - Data Erratic
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Overview
Fault Code
SPN 2631 / FMI 2
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: EGR mass flow sensing components are typically located on the EGR valve assembly or inlet manifold, near the top of the engine on the intake side.
Common Symptoms
- Active fault code SPN 2631 FMI 2 logged in ECM
- Erratic or fluctuating EGR flow readings on diagnostic software
- Rough idle or unstable engine operation
- Increased NOx emissions or failed emissions test
- Reduced fuel economy
- DPF regeneration frequency increased
- Possible black or excessive smoke from exhaust
- Engine may exhibit hunting or surging at low RPM
Root Causes
- Plugged or carbon-fouled EGR valve causing erratic flow Failed or dirty EGR differential pressure sensor (delta-P sensor) Carbon buildup in EGR cooler restricting flow inconsistently Faulty or contaminated EGR mass flow sensor Cracked, collapsed, or leaking EGR pipe or hose causing intermittent flow loss ECM calibration or software fault producing erratic signal interpretation EGR valve stuck partially open or closing intermittently Wiring harness damage, chafing, or loose connector at EGR sensor or valve Failed EGR cooler causing coolant intrusion affecting sensor signal Intake manifold pressure sensor fault affecting EGR flow calculation
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Derate Behavior
Mileage / Regen Notes
SPN 2631 FMI 2 may inhibit or interrupt active DPF regeneration cycles due to unreliable EGR flow data affecting combustion control. Passive regen may also be disrupted if EGR mass flow errors cause improper air-fuel ratio management, leading to increased soot loading.
System Interactions
SPN 2631 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: EGR sensor TSB
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 2631 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.
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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