SPN 168 FMI 0Electrical/J1939High
Battery Voltage — High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 168 / FMI 0
Category
Electrical/J1939
Fault Type
sensor
FMI Category
voltage high
Severity
High
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Alternator — engine-mounted; battery — battery box
Common Symptoms
- High battery voltage
- Electrical component damage possible
- Alternator overcharging
- Battery overheating
Root Causes
- Alternator voltage regulator failed
- Battery temperature sensor failed
- Charging system fault
- Alternator overcharging
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Alternator voltage regulator failure common at 400k–600k miles.
System Interactions
SPN 168 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021101 — Alternator Overcharging 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
X15: Overcharging can damage ECM and other electronics.
Detroit
DD15: Check battery temperature sensor before condemning alternator.
PACCAR
MX-13: Check for aftermarket accessories causing charging issues.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 168 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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