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Fault Code Reference

SPN 2791 FMI 0AftertreatmentInducement Risk

Aftertreatment DPF Outlet Temperature - Data Valid But Above Normal

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Overview

Fault Code

SPN 2791 / FMI 0

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DPF outlet temperature sensor is mounted on the exhaust pipe exiting the downstream face of the DPF filter canister, typically in the aftertreatment assembly under or behind the cab.

Common Symptoms

  • DPF outlet temperature sensor reading extremely high
  • Active regeneration temperature exceeding limits
  • Check engine light illuminated
  • High exhaust system indicator lamp active
  • Excessive heat from exhaust system
  • DPF face plugging or restriction present

Root Causes

  • Actual DPF outlet temperature exceeding normal range due to uncontrolled or runaway regeneration
  • Faulty or stuck-open dosing injector causing excess fuel in exhaust stream
  • Incorrect fuel dosing quantity from ACM calibration or software issue
  • Restricted or failed DPF causing excessive heat buildup during regen
  • Faulty DPF outlet temperature sensor reporting falsely high values
  • Failed or malfunctioning DPF outlet temperature sensor wiring short to voltage
  • Low exhaust flow condition causing concentrated heat at DPF outlet
  • Incorrect soot load estimation leading to over-regeneration

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

SPN 2791 FMI 0 will typically inhibit or abort active DPF regeneration to prevent thermal damage. Repeated faults may result in regen lockout and progressive derate until root cause is resolved.

System Interactions

SPN 2791 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components

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.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 2791 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.

EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx 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.

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