SPN 177 FMI 0DrivetrainHigh
Transmission Oil Temperature — High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 177 / FMI 0
Category
Drivetrain
Fault Type
sensor
FMI Category
performance
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Transmission oil temperature sensor — on transmission; transmission cooler — radiator or separate cooler
Common Symptoms
- Transmission oil temperature high
- Transmission derate
- Transmission shifting issues
- Possible transmission damage
Root Causes
- Transmission cooler restriction
- Low transmission fluid level
- Heavy load operation
- Transmission cooler bypass valve stuck
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Transmission cooler restriction common at 300k–500k miles.
System Interactions
SPN 177 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Allison TSB 2019-012 — Transmission Oil Temperature High 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 transmission cooler for restriction.
Eaton
Check transmission fluid level first.
Meritor
Check cooler bypass valve.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 177 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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