SPN 1323 FMI 1Fuel System
High Pressure Fuel Pump Inlet Pressure - Data Valid But Below Normal
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Overview
Fault Code
SPN 1323 / FMI 1
Category
Fuel System
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: The high pressure fuel pump inlet pressure sensor is located on the fuel inlet side of the high pressure common rail fuel pump mounted on the engine block.
Common Symptoms
- Engine cranks but fails to start or hard starting
- Loss of power under load or acceleration
- Intermittent engine stalling or shutdown
- Rough running or misfiring at idle
- Engine derate or reduced performance
- Increased white smoke from exhaust during cold start
Root Causes
- Clogged or restricted fuel filter Low fuel level or fuel starvation condition Faulty or failing low pressure fuel pump Air ingestion in the fuel supply system Restricted fuel supply line or collapsed fuel hose
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Derate Behavior
Power Derate
Yes
System Interactions
SPN 1323 FMI 1 interacts with the following systems and may trigger cascading faults if not resolved promptly.
Fuel rail pressure — injector timing and quantity affected
Engine power output — derate may be triggered to protect injectors
Aftertreatment regen — low fuel pressure can prevent regen completion
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.
Replacing injectors without verifying fuel supply pressure — low supply pressure causes injector faults.
Ignoring fuel filter restriction — a clogged filter causes low rail pressure that mimics injector failure.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1323 FMI 1 — 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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