SPN 158 FMI 4Electrical
Battery Potential / Power Input 1 - Voltage Below Normal or Shorted Low
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 158 / FMI 4
Category
Electrical
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Battery voltage sensing circuit is monitored at the ECM power supply input, originating from the main battery terminals.
Common Symptoms
- Low voltage warning light illuminated
- ECM logging intermittent power loss events
- Engine cranks slowly or fails to start
- Multiple fault codes present across modules
- Instrument cluster gauges fluctuating or dimming
- Loss of communication with ECM
- Engine stalling at idle or under load
Root Causes
- Discharged or weak batteries Corroded or loose battery cable connections Faulty alternator not charging properly Damaged wiring harness between alternator and ECM High parasitic draw draining batteries
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Derate Behavior
Power Derate
No
System Interactions
SPN 158 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
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 158 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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