SPN 121 FMI 0ABS
ABS Control Module Communication Lost
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 121 / FMI 0
Category
ABS
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: ABS control module is typically mounted on the frame rail or chassis near the air brake system components.
Common Symptoms
- ABS warning lamp illuminated on dash
- Traction control or stability control warning lamp on
- ABS system defaults to non-ABS braking operation
- Loss of communication fault logged in ECM or BCM diagnostic scan
- Intermittent or complete loss of ABS fault codes during active driving
- No communication with ABS module via diagnostic scan tool
Root Causes
- Loss of power or ground to ABS control module causing communication dropout on J1939 databus
- Corroded, damaged, or loose CAN bus termination resistor (120-ohm) causing network instability
- Failed or failing ABS ECU internal hardware preventing module from transmitting on J1939
- Damaged or shorted CAN High/CAN Low wiring harness between ABS module and backbone
- J1939 datalink backbone splice or connector corrosion causing intermittent open circuit
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Active DPF regeneration will be inhibited or suspended when ABS communication is lost, as the aftertreatment controller requires confirmed wheel speed and vehicle speed data from ABS to safely authorize regen; parked regen may also be blocked.
System Interactions
SPN 121 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 121 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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