SPN 1209 FMI 31TurbochargerMedium
Engine Turbocharger 1 Compressor Intake Pressure — Condition Exists
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1209 / FMI 31
Category
Turbocharger
Fault Type
sensor
FMI Category
condition exists
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Compressor inlet pressure sensor on air intake duct — between air filter and turbocharger compressor inlet.
Common Symptoms
- Turbocharger compressor inlet pressure condition active
- Possible air restriction warning
- Boost pressure monitoring affected
- May accompany air filter restriction fault
Root Causes
- Air filter restriction — plugged filter creating excessive inlet vacuum
- Air intake duct collapsed or kinked
- Turbocharger compressor inlet pressure sensor fault
- Pre-cleaner plugged
- Air intake duct disconnected or leaking
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Maintenance-driven. Higher frequency in dusty or construction environments.
System Interactions
SPN 1209 FMI 31 interacts with the following systems and may trigger cascading faults if not resolved promptly.
Boost pressure — engine air/fuel ratio and power output affected
EGR system — boost pressure affects EGR flow calculations
Aftertreatment — low boost can cause incomplete combustion and DPF loading
TSB Reference: OEM PM schedule — air filter inspection and replacement procedure
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: FMI 31 (condition exists) is a general fault. Check air filter restriction first — most common cause.
Detroit
DD15: Inspect air intake pre-cleaner at every PM in dusty environments.
PACCAR
MX-13: Check air intake duct coupler at turbo inlet — known failure point causing air restriction.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1209 FMI 31 — 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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