SPN 168 FMI 2Electrical/J1939High
Battery Voltage — Erratic / Intermittent
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 168 / FMI 2
Category
Electrical/J1939
Fault Type
electrical
FMI Category
data erratic
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Batteries — typically frame rail, driver side. Ground straps connect battery negative to chassis and engine block.
Common Symptoms
- Battery voltage reading erratic in live data
- Intermittent electrical faults across multiple systems
- Lights flickering or dimming intermittently
- ECM resets or communication dropouts
- Multiple fault codes setting and clearing intermittently
Root Causes
- Loose or corroded battery cable connections
- Battery internal failure — cell shorting intermittently
- Alternator output voltage erratic
- Battery voltage sense wire loose or corroded
- Ground strap loose or corroded — chassis to engine or battery
- Parasitic draw causing battery voltage to drop intermittently
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Battery failure common at 3–5 years regardless of mileage. Cable corrosion increases with age.
System Interactions
SPN 168 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021812 — Charging System and Battery Diagnosis Procedure
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
X15: Erratic battery voltage often caused by corroded battery cable ends. Clean with wire brush and apply dielectric grease.
Detroit
DD15: Check smart alternator communication — erratic voltage can cause DD15 to set multiple unrelated faults.
PACCAR
MX-13: Inspect battery box for moisture — water pooling in battery box accelerates terminal corrosion.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 168 FMI 2 — 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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