SPN 158 FMI 1Electrical
Battery Potential / Power Input 1 - Data Valid But Below Normal
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 158 / FMI 1
Category
Electrical
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Battery pack is typically located on the vehicle frame rail, with voltage sensing at the ECM power supply input.
Common Symptoms
- Low battery voltage warning light illuminated
- Hard starting or slow cranking
- ECM logging low voltage events
- Intermittent electrical system faults
- Multiple electronic control modules reporting low voltage
- Dimming lights or weak accessories operation
- Check engine light or warning lamp active
Root Causes
- Discharged or weak batteries Faulty alternator or charging system malfunction Corroded or loose battery cable connections Parasitic electrical draw draining batteries Damaged battery voltage sensor circuit
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Derate Behavior
Power Derate
No
System Interactions
SPN 158 FMI 1 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.
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OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 158 FMI 1 — 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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