SPN 91 FMI 4Engine PerformanceHigh
Throttle Position — Voltage Below Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 91 / FMI 4
Category
Engine Performance
Fault Type
sensor
FMI Category
electrical low
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Throttle pedal assembly — cab floor, driver side
Common Symptoms
- Throttle pedal not responding or limited response
- Engine stuck at idle or low RPM
- Throttle position reading 0% or very low in live data
- Possible limp mode
Root Causes
- Throttle position sensor wiring short to ground
- Throttle pedal assembly failure
- Damaged throttle pedal connector
- Open circuit in throttle signal wire
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Pedal assembly failures common 400k–600k miles. Connector corrosion can occur at any mileage.
System Interactions
SPN 91 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.
ECM communication — J1939 network faults can affect all connected modules
Engine protection — ECM may trigger derate to prevent damage
Aftertreatment — engine protection derates can affect regen strategy
TSB Reference: Cummins TSB 4021529 — Throttle Position 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
Cummins
ISX15: Throttle pedal is a sealed assembly — replace as unit if sensor fails.
Detroit
DD15: Check for moisture intrusion in pedal connector — common in cab floor area.
PACCAR
MX-13: Verify both throttle sensor circuits — ECM monitors both for rationality.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 91 FMI 4 — 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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