SPN 2791 FMI 13AftertreatmentInducement Risk
NOx Sensor 1 - Out Of Calibration
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Overview
Fault Code
SPN 2791 / FMI 13
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: NOx sensor 1 is located upstream of the SCR catalyst in the exhaust aftertreatment system, typically mounted on the DOC/DPF outlet or SCR inlet pipe.
Common Symptoms
- Check engine light illuminated
- Engine derate or power reduction
- Increased DEF consumption
- Emissions system fault message displayed
- NOx sensor malfunction indicator
- Possible failure to meet emissions compliance
Root Causes
- NOx sensor 1 calibration data mismatch or corruption in ACM memory NOx sensor 1 failed internally (degraded sensing element) Exhaust leaks upstream of NOx sensor 1 causing inaccurate readings Incorrect NOx sensor installed (part number mismatch) ACM software or calibration file out of date Contaminated or damaged NOx sensor connector causing signal offset EGR system malfunction causing abnormal NOx levels skewing calibration check
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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
An out-of-calibration NOx Sensor 1 fault (FMI 13) may inhibit active DPF regeneration and can trigger DPF regen lockout as the ACM relies on accurate upstream NOx data for aftertreatment system control. SCR dosing accuracy may also be compromised, potentially leading to additional SCR/DEF faults and derate conditions.
System Interactions
SPN 2791 FMI 13 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 2791 FMI 13 — 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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