SPN 1809 FMI 31Drivetrain
Transmission Output Shaft Speed Sensor - Condition Exists
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1809 / FMI 31
Category
Drivetrain
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The output shaft speed sensor is mounted on the transmission housing near the output shaft where it reads a tone ring or reluctor wheel.
Common Symptoms
- Transmission will not shift properly or stays in one gear
- Speedometer inoperative or erratic
- Check engine light or transmission warning light illuminated
- Vehicle may enter limp mode with reduced power
- Cruise control inoperative
- ABS or traction control warnings may also appear
Root Causes
- Faulty output shaft speed sensor Damaged or corroded sensor wiring harness Intermittent sensor signal due to debris or contamination on sensor face Failed transmission control module Improper sensor air gap or loose mounting
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Derate Behavior
Power Derate
No
System Interactions
SPN 1809 FMI 31 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1809 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.
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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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