SPN 731 FMI 0Transmission
Incorrect Gear Ratio - Gear 1
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 731 / FMI 0
Category
Transmission
Derate Risk
Possible
Regen Inhibit
No
Inducement Risk
No
Component Location: Transmission assembly mounted to rear of engine, with speed sensors and TCM typically located on or near the transmission housing or crossmember.
Common Symptoms
- Transmission fails to engage or slips in 1st gear
- Harsh or delayed 1st gear engagement
- Transmission warning lamp illuminated on dash
- Vehicle may default to limp-home mode restricting gear range
- Abnormal noise or shudder during 1st gear operation
- Diagnostic trouble code logged with no-start or limited acceleration in low range
Root Causes
- Transmission internal mechanical wear or damage causing incorrect gear ratio in first gear (e.g., worn clutch packs, damaged planetary gears) Faulty or out-of-range transmission output or input speed sensor providing incorrect ratio data to TCM Low or degraded transmission fluid causing slippage and incorrect ratio detection TCM software fault or corrupted calibration data causing erroneous gear ratio calculation Damaged or slipping torque converter affecting ratio calculations in lower gear ranges
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Derate Behavior
Power Derate
Possible
System Interactions
SPN 731 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
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 731 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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