SPN 1079 FMI 3Electrical/J1939High
Sensor Supply Voltage 1 — Voltage Above Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1079 / FMI 3
Category
Electrical/J1939
Fault Type
sensor
FMI Category
voltage high
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: 5V reference circuit — ECM to all sensors using 5V reference
Common Symptoms
- Multiple sensor faults active
- 5V reference circuit fault
- Multiple sensors reading high
- Engine derate
Root Causes
- 5V reference circuit shorted to battery
- ECM internal fault
- Wiring short to power affecting multiple sensors
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Wiring shorts can occur at any mileage. ECM failure less common.
System Interactions
SPN 1079 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021220 — 5V Reference Circuit Short High 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: 5V reference short high causes multiple sensor faults.
Detroit
DD15: DDDL shows which sensors are affected.
PACCAR
MX-13: Check wiring harness for chafing near hot surfaces.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1079 FMI 3 — 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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