SPN 168 FMI 4ElectricalHigh
Battery Voltage — Voltage Below Normal (Electrical)
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 168 / FMI 4
Category
Electrical
Fault Type
sensor
FMI Category
electrical low
Severity
High
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: ECM — mounted on engine or firewall depending on OEM. Battery box on frame rail.
Common Symptoms
- Battery voltage reading critically low in ECM
- Multiple systems going offline
- Possible ECM reset or memory loss
Root Causes
- Battery voltage below ECM minimum threshold
- Loose or corroded battery cable at ECM power supply
- Failed battery
- Alternator failure
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Wiring faults can occur at any mileage. Battery failures common 3–5 years.
System Interactions
SPN 168 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 4021539 — ECM Power Supply 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: Check ECM power supply fuse (typically 30A) and relay before condemning batteries.
Detroit
DD15: ECM ground strap condition is critical — check all ground points.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 168 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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