What are the symptoms of SPN 1677 FMI 31?

Check engine light illuminated. Engine derate or power reduction. Poor DEF consumption or unexpectedly high DEF usage. Increased NOx emissions. SCR system efficiency warning on dashboard

What causes SPN 1677 FMI 31?

Insufficient or degraded DEF (diesel exhaust fluid) quality — diluted, contaminated, or crystallized DEF DEF dosing system malfunction — clogged or faulty DEF injector not delivering correct urea quantity SCR catalyst degraded, poisoned (sulfur, phosphorus, oil ash), or thermally damaged DEF supply module or pump failure resulting in incorrect dosing rate NOx sensor (downstream) failure giving false low conversion efficiency reading NOx sensor (upstream) failure skewing efficiency ratio calculation DEF tank level sensor fault causing incorrect dosing commands Exhaust system leak upstream or downstream of SCR catalyst affecting NOx measurement ECM/ACM software calibration error or incorrect NOx sensor learned values DEF lines frozen, kinked, or restricted causing reduced DEF flow

How do I diagnose SPN 1677 FMI 31?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 1677 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 1677 FMI 31AftertreatmentInducement Risk

Aftertreatment 1 SCR Conversion Efficiency - Condition Exists

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Overview

Fault Code

SPN 1677 / FMI 31

Common Symptoms

Check engine light illuminated
Engine derate or power reduction
Poor DEF consumption or unexpectedly high DEF usage
Increased NOx emissions
SCR system efficiency warning on dashboard

Root Causes

Insufficient or degraded DEF (diesel exhaust fluid) quality — diluted, contaminated, or crystallized DEF DEF dosing system malfunction — clogged or faulty DEF injector not delivering correct urea quantity SCR catalyst degraded, poisoned (sulfur, phosphorus, oil ash), or thermally damaged DEF supply module or pump failure resulting in incorrect dosing rate NOx sensor (downstream) failure giving false low conversion efficiency reading NOx sensor (upstream) failure skewing efficiency ratio calculation DEF tank level sensor fault causing incorrect dosing commands Exhaust system leak upstream or downstream of SCR catalyst affecting NOx measurement ECM/ACM software calibration error or incorrect NOx sensor learned values DEF lines frozen, kinked, or restricted causing reduced DEF flow

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 1677 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

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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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