SPN 1798 FMI 0Suspension
Lift Axle Control Switch - Data Valid But 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 1798 / FMI 0
Category
Suspension
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The lift axle control switch is typically mounted in the dash or on the center console within reach of the driver.
Common Symptoms
- Lift axle will not deploy or retract
- Lift axle stuck in raised position
- Dash warning light for lift axle control
- Intermittent lift axle operation
- BCM fault code active
Root Causes
- Faulty lift axle control switch with internal short to high voltage Wiring harness damage causing short to battery voltage Damaged or corroded switch connector terminals Control module internal fault reading incorrect high signal Incorrect switch installation or mechanical binding
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Derate Behavior
Power Derate
No
System Interactions
SPN 1798 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1798 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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