SPN 1127 FMI 1Turbo/AirMedium
Engine Turbocharger 1 Compressor Inlet Pressure — Low
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1127 / FMI 1
Category
Turbo/Air
Fault Type
sensor
FMI Category
performance
Severity
Medium
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Air intake system — air filter housing, intake hose, turbo inlet
Common Symptoms
- Air filter restriction indicated
- Reduced turbo performance
- Possible derate
- Black smoke under load
Root Causes
- Air filter severely restricted
- Intake air restriction upstream of turbo
- Collapsed intake hose
- Sensor reading low due to restriction
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Air filter restriction common at 100k–150k miles under dusty conditions.
System Interactions
SPN 1127 FMI 1 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021503 — Air Intake Restriction 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: Air filter service interval 100k miles or per restriction indicator.
Detroit
DD15: Check air filter housing seal for bypass.
PACCAR
MX-13: Inspect intake hose clamps for looseness.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1127 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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