AI Diesel Tech

Fault Code Reference

SPN 2791 FMI 9EGR

EGR Valve Control 1 - Abnormal Update Rate

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Overview

Fault Code

SPN 2791 / FMI 9

Category

EGR

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

No

Component Location: EGR valve actuator is mounted on the EGR valve body on the intake side of the engine, typically near the intake manifold or EGR cooler outlet, connected via CAN bus and power harness.

Common Symptoms

  • Active fault code SPN 2791 FMI 9 logged in ECM
  • EGR valve position may be fixed or unresponsive
  • Reduced engine performance or derate condition
  • Increased NOx emissions or failed emissions test
  • Engine may run rough or exhibit unstable idle
  • DPF regeneration frequency may increase
  • CAN bus communication faults may accompany this code

Root Causes

  • CAN bus communication failure between EGR valve controller and ECM Faulty or damaged J1939 CAN harness wiring (open, short, or high resistance) EGR valve actuator module internal failure causing loss of message broadcast Corroded or damaged connector at EGR valve or ECM Terminating resistor failure on CAN bus (should be 60 ohms across CAN H/L with ignition off) ECM software or calibration issue causing message timeout Power or ground supply fault to EGR valve actuator module Multiple device CAN bus loading issue (excessive nodes dropping message rate)

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Derate Behavior

Power Derate

Yes

System Interactions

SPN 2791 FMI 9 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 2791 FMI 9 — 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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