SPN 123 FMI 0ABS/BrakesHigh
Wheel Speed Sensor Circuit Range/Performance (Left Rear)
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 123 / FMI 0
Category
ABS/Brakes
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: The left rear wheel speed sensor is mounted on the axle housing near the left rear wheel hub assembly.
Common Symptoms
- ABS warning lamp illuminated on dash
- ABS system disabled or inactive
- Traction control malfunction warning
- Stability control system disabled
- Speedometer erratic or inaccurate reading
Root Causes
- Faulty left rear wheel speed sensor Damaged or corroded sensor wiring harness Excessive air gap between sensor and tone ring Worn or damaged tone ring teeth Water or debris contamination at sensor connector
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System Interactions
SPN 123 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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Deep-dive diagnostic page: SPN 123 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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