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Fault Code Reference

SPN 2791 FMI 2AftertreatmentInducement Risk

NOx Sensor 1 Heater - Data Erratic, Intermittent or Incorrect

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Overview

Fault Code

SPN 2791 / FMI 2

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: NOx sensor 1 is located in the exhaust stream upstream of the SCR catalyst, typically mounted on the exhaust pipe between the DPF outlet and SCR inlet.

Common Symptoms

  • Check engine light illuminated
  • Engine derate or reduced power
  • SCR system efficiency fault codes present
  • Exhaust aftertreatment warning lamp active
  • Poor fuel economy
  • Elevated NOx emissions

Root Causes

  • NOx sensor 1 heater circuit intermittent open or short due to chafed or damaged wiring harness Corroded or moisture-contaminated NOx sensor connector causing erratic heater signal Faulty NOx sensor 1 with internal heater element failure Poor or loose ground connection at NOx sensor or ACM ground circuit ACM software fault or calibration issue causing erratic heater data interpretation Excessive exhaust condensation damaging sensor heater element prematurely Water intrusion into NOx sensor connector

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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 2 can inhibit or delay active DPF regeneration as the ACM relies on accurate NOx sensor 1 data to manage aftertreatment; erratic heater data may trigger regen inhibit or derate conditions depending on OEM thresholds.

System Interactions

SPN 2791 FMI 2 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 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.

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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