What are the symptoms of SPN 2791 FMI 4?

Check engine light illuminated. EGR valve position sensor reading at or near 0% regardless of commanded position. EGR valve not responding to commands during key-on engine-off testing. Possible rough idle or elevated NOx emissions. Engine may run warmer than normal due to lack of EGR flow. Scan tool live data shows EGR position voltage below 0.2V or stuck at minimum value. Occasional black smoke under load if EGR is stuck open mechanically but sensor reads low.

What causes SPN 2791 FMI 4?

[object Object]. [object Object]. [object Object]. [object Object]. [object Object]. [object Object]

How do I diagnose SPN 2791 FMI 4?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 2791 FMI 4 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 2791 FMI 4EGRMedium

EGR Valve Position - Voltage Low

Run Full Diagnosis View Related Faults See OEM System Guide

Scan Tool — in guided session

DVOM Testing — in guided session

Connector Pinout — in guided session

DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.

Overview

Fault Code

SPN 2791 / FMI 4

Common Symptoms

Check engine light illuminated. EGR valve position sensor reading at or near 0% regardless of commanded position. EGR valve not responding to commands during key-on engine-off testing. Possible rough idle or elevated NOx emissions. Engine may run warmer than normal due to lack of EGR flow. Scan tool live data shows EGR position voltage below 0.2V or stuck at minimum value. Occasional black smoke under load if EGR is stuck open mechanically but sensor reads low.

Root Causes

[object Object]
[object Object]
[object Object]
[object Object]
[object Object]
[object Object]

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

Start Guided Diagnosis See How It Works

Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault alone typically does not inhibit active DPF regeneration. However, if the ECM cannot accurately control EGR position, exhaust temperatures and air-fuel ratios may be affected, which can indirectly cause regen quality issues or extended regen times. If combined with other aftertreatment faults, the system may postpone or inhibit active regen until the EGR fault is resolved. Monitor for secondary codes related to regen efficiency if this fault is present for extended operation.

System Interactions

SPN 2791 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 had service letters regarding EGR harness chafing near turbo heat shields. Detroit DD13/DD15 have documented connector corrosion issues at EGR valve. Paccar MX engines have had TSBs for EGR wiring routing causing chafe points. Always check OEM-specific bulletins for applicable engine family.

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

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3251-0DPF Differential Pressure High SPN 157-18Fuel Rail Pressure Low

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

Diagnostic Engine v7.0

Diagnose SPN 2791 FMI 4 Now Start New Diagnosis

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.

Get weekly diesel diagnostic tips

Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.

No spam. Unsubscribe anytime.

Get AI-Guided Diagnosis for SPN 2791 FMI 4

The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.