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Fault Code Reference

SPN 2791 FMI 31AftertreatmentInducement Risk

Diesel Exhaust Fluid Heater - Condition Exists

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Overview

Fault Code

SPN 2791 / FMI 31

Category

Aftertreatment

Derate Risk

Possible

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF heater elements are located in the DEF tank and along the DEF supply lines, typically mounted at or near the rear frame rail on the driver side.

Common Symptoms

  • DEF heater system malfunction warning
  • Poor DEF dosing performance in cold weather
  • DEF system frozen or slow to thaw
  • SCR efficiency reduced in cold ambient conditions
  • DEF quality or temperature sensor faults may accompany
  • Reduced NOx conversion efficiency

Root Causes

  • DEF heater circuit failure or open/short in heater element wiring Failed or burnt-out DEF tank heater element DEF supply line heater failure preventing thawing in cold temperatures Faulty DEF heater relay or blown fuse in heater circuit Poor ground connection at DEF heater or ACM ACM software fault or calibration mismatch requiring reflash Failed DEF pump module with integrated heater circuit Corrosion or moisture intrusion at DEF heater connector

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Derate Behavior

Power Derate

Possible

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 31 may inhibit or delay active DPF regeneration if the DEF system is deemed non-functional, particularly in cold ambient conditions where DEF thawing cannot be confirmed. Some ACM calibrations will defer regen until DEF heater functionality is verified to prevent urea crystallization.

System Interactions

SPN 2791 FMI 31 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 31 — 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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