AI Diesel Tech

Fault Code Reference

SPN 1761 FMI 31AftertreatmentInducement Risk

Aftertreatment DEF Doser - Condition Exists

Run Full DiagnosisView Related FaultsSee OEM System Guide

Scan Tool — in guided session

DVOM Testing — in guided session

Connector Pinout — in guided session

DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.

Overview

Fault Code

SPN 1761 / FMI 31

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF doser valve is mounted on the DEF decomposition tube or mixing pipe downstream of the DPF and upstream of the SCR catalyst, typically on the exhaust mid-pipe.

Common Symptoms

  • Amber DEF system warning lamp illuminated
  • DEF consumption higher or lower than expected
  • DEF doser injecting continuously or not at all
  • Poor NOx conversion efficiency
  • Ammonia slip detected at SCR outlet
  • DEF system performance degraded message

Root Causes

  • DEF doser valve clogged or crystallized with urea deposits
  • DEF supply line restriction or blockage
  • DEF quality issue or contaminated DEF fluid
  • DEF doser electrical connector corrosion or poor connection
  • Failed or stuck DEF doser valve (open or closed)
  • DEF pump pressure out of specification
  • ACM software or calibration issue requiring update
  • DEF tank level sensor fault causing incorrect dosing logic
  • Air in DEF supply lines
  • DEF injector tip damaged or eroded from heat exposure

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

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

Start Guided DiagnosisSee How It Works

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 1761 FMI 31 indicates a DEF dosing condition exists which will inhibit or severely degrade SCR system performance. Active and passive DPF regeneration may be inhibited or incomplete if DEF dosing faults are present, as the SCR system cannot properly reduce NOx. Continued operation may result in SCR deactivation, regen lockout, and derate progression.

System Interactions

SPN 1761 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 1761 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.

Get weekly diesel diagnostic tips

Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.

No spam. Unsubscribe anytime.

Get AI-Guided Diagnosis for SPN 1761 FMI 31

The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.

Run Full DiagnosisView Related FaultsSee OEM System Guide