SPN 1438 FMI 4ABS
Front Axle Service Brake Pressure - Voltage Below Normal
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Connector Pinout — in guided session
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Overview
Fault Code
SPN 1438 / FMI 4
Category
ABS
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Front axle brake pressure sensor is typically mounted on the front axle modulator valve or brake chamber inlet port.
Common Symptoms
- ABS warning light illuminated
- Tractor protection valve may activate
- Loss of ABS functionality
- Brake pressure gauge reading low or zero
- Possible service brake application issues
Root Causes
- Faulty front axle brake pressure sensor Open or damaged wiring harness to front axle pressure sensor Corroded or loose connector at pressure sensor Low supply voltage to pressure sensor Damaged ABS module or ECU internal fault
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Derate Behavior
Power Derate
No
System Interactions
SPN 1438 FMI 4 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 1438 FMI 4 — 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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