SPN 191 FMI 2DrivetrainHigh
Transmission Output Shaft Speed — Erratic
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 191 / FMI 2
Category
Drivetrain
Fault Type
sensor
FMI Category
data erratic
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Transmission output speed sensor — on transmission output shaft
Common Symptoms
- Transmission output speed sensor erratic
- Transmission shifting issues
- Speedometer erratic
- ABS faults
Root Causes
- Output speed sensor failed
- Tone ring damaged
- Wiring harness damage
- Connector corrosion
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
Mileage / Regen Notes
Speed sensor failure common at 200k–400k miles.
System Interactions
SPN 191 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Allison TSB 2019-014 — Output Speed Sensor Diagnosis
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.
OEM Notes
Allison
Check tone ring for damage before replacing sensor.
Eaton
Check sensor air gap.
Meritor
Check connector for corrosion.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 191 FMI 2 — 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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