SPN 3609 FMI 0Aftertreatment
Aftertreatment 1 Intake Dew Point - Data Valid But Above Normal
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3609 / FMI 0
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The intake dew point sensor is mounted in the air intake system upstream of the turbocharger compressor inlet.
Common Symptoms
- Aftertreatment intake dew point reading abnormally high
- ACM/ECM communication warnings or fault messages
- Check engine light illuminated
- Reduced regeneration efficiency or incomplete regens
- Excessive moisture detected in aftertreatment intake air
- DPF face plugging complaints
Root Causes
- Intake air temperature and humidity sensor failure or drift Excessive ambient humidity conditions beyond normal range Charge air cooler (CAC) condensation or water contamination Air intake system restrictions causing high humidity buildup Sensor calibration drift or out-of-range condition Aftertreatment intake air temperature sensor fault Sensor wiring harness damage, corrosion, or moisture intrusion ACM internal calculation or software error
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Derate Behavior
Power Derate
No
System Interactions
SPN 3609 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components
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 the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.
Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3609 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.
EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx 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.
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