SPN 571 FMI 0ABS
Brake Switch A Circuit
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 571 / FMI 0
Category
ABS
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Brake light switch is mounted on the brake pedal bracket assembly in the cab, near the top of the brake pedal arm under the dash.
Common Symptoms
- Brake lights remain on continuously with no pedal input
- ABS warning lamp illuminated on dash
- Cruise control will not engage or disengages unexpectedly
- Engine brake (Jake brake) inhibited or non-functional
- Transmission shift strategy altered or hold in current gear
- Higher than expected voltage signal from brake switch circuit
Root Causes
- Brake light switch mechanically out of adjustment or worn, causing continuous or erratic signal output
- Brake light switch internal electrical failure (contacts welded closed or open circuit under load)
- Wiring harness chafing or connector corrosion at brake switch connector causing signal voltage out of range
- Brake pedal assembly wear or damage affecting proper switch actuation point
- ABS/ECM module input circuit fault misreading switch signal voltage
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 571 FMI 0 (brake switch high signal) can inhibit or interrupt active DPF regeneration cycles, as most aftertreatment controllers interpret a continuous brake signal as a stop event, halting parked or active regen to prevent thermal risk.
System Interactions
SPN 571 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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 571 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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