SPN 521 FMI 2Brakes
ABS Axle 1 Right Wheel Speed Sensor - Erratic/Intermittent
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 521 / FMI 2
Category
Brakes
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The right wheel speed sensor for axle 1 is mounted at the wheel end hub assembly, typically on the steer axle.
Common Symptoms
- ABS warning lamp illuminated
- Traction control disabled
- Stability control disabled
- Erratic speedometer readings
- ABS system inactive
- Cruise control inoperative
Root Causes
- Damaged or contaminated wheel speed sensor tone ring Faulty wheel speed sensor or internal circuit failure Damaged wiring harness or connector corrosion at the wheel speed sensor Excessive bearing play or wheel end looseness Debris or metallic buildup between sensor and tone ring
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Derate Behavior
Power Derate
No
System Interactions
SPN 521 FMI 2 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 521 FMI 2 — 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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