SPN 191 FMI 0TransmissionMedium
Gear Shift Error
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 0
Category
Transmission
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: Transmission output shaft speed sensor and TCM are located on or adjacent to the transmission housing, typically mounted mid-chassis beneath the cab.
Common Symptoms
- Transmission fails to shift or shifts erratically
- Dash warning light or transmission fault lamp illuminated
- Vehicle stuck in a single gear or limp-home mode
- Harsh or delayed gear engagement
- Reduced vehicle speed or power derate active
- Driver unable to manually select gears via shifter
Root Causes
- Output shaft speed sensor fault or signal out of range
- Transmission control module (TCM) internal fault or software issue
- Mechanical gear selection linkage wear or misadjustment
- Wiring harness damage or connector corrosion at TCM or speed sensor
- Low or contaminated transmission fluid causing slippage
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Derate Behavior
Mileage / Regen Notes
Active SPN 191 FMI 0 may inhibit or interrupt DPF regeneration cycles due to derate conditions or inability to maintain required vehicle speed and load for regen completion.
System Interactions
SPN 191 FMI 0 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 191 FMI 0 — 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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