What are the symptoms of SPN 1079 FMI 31?

Check engine light/MIL illuminated. SCR system efficiency fault codes present. NOx conversion efficiency below threshold. Elevated NOx emissions readings. DEF consumption appears abnormal. Downstream NOx sensor reading implausible or erratic

What causes SPN 1079 FMI 31?

SCR outlet NOx sensor failed or degraded (sensor poisoning or aging) NOx sensor heater circuit fault causing incorrect readings SCR catalyst efficiency below threshold (catalyst degradation or plugging) DEF quality issue or urea deposit causing incomplete SCR conversion DEF dosing system fault (doser clogged, pump failure) leading to under-dosing Exhaust air or exhaust leak upstream affecting NOx sensor accuracy NOx sensor connector corrosion or poor pin contact ACM software or calibration issue misinterpreting sensor signal

How do I diagnose SPN 1079 FMI 31?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 1079 FMI 31 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 1079 FMI 31AftertreatmentInducement Risk

SCR Outlet NOx Sensor - Condition Exists

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Overview

Fault Code

SPN 1079 / FMI 31

Common Symptoms

Check engine light/MIL illuminated
SCR system efficiency fault codes present
NOx conversion efficiency below threshold
Elevated NOx emissions readings
DEF consumption appears abnormal
Downstream NOx sensor reading implausible or erratic

Root Causes

SCR outlet NOx sensor failed or degraded (sensor poisoning or aging) NOx sensor heater circuit fault causing incorrect readings SCR catalyst efficiency below threshold (catalyst degradation or plugging) DEF quality issue or urea deposit causing incomplete SCR conversion DEF dosing system fault (doser clogged, pump failure) leading to under-dosing Exhaust air or exhaust leak upstream affecting NOx sensor accuracy NOx sensor connector corrosion or poor pin contact ACM software or calibration issue misinterpreting sensor signal

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

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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.

System Interactions

SPN 1079 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.

Related Faults in This Category

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3364-2DEF Quality Erratic SPN 3251-0DPF Differential Pressure High SPN 3719-16DPF Soot Load High

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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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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