SPN 3609 FMI 2AftertreatmentInducement Risk
Exhaust Gas Temperature Bank 1 Sensor 4 - Data Erratic
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Overview
Fault Code
SPN 3609 / FMI 2
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: Bank 1 Sensor 4 is typically located in the aftertreatment system outlet, downstream of the SCR catalyst and DPF.
Common Symptoms
- Amber check engine light illuminated
- DPF regeneration issues or frequent regens
- Poor fuel economy
- Black smoke from exhaust
- Reduced engine power in some cases
Root Causes
- Failed or degraded EGT sensor 4 (Bank 1) producing erratic/irrational signal
- Corroded or damaged connector at EGT sensor 4 causing intermittent signal
- Chafed, shorted, or open wiring harness between EGT sensor 4 and ACM
- Exhaust gas temperature sensor 4 contaminated by soot or moisture intrusion
- Loose or improperly seated sensor in bung causing thermal measurement error
- ACM software issue or internal ACM fault causing erratic sensor reading
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Derate Behavior
Power Derate
Yes
SCR Inducement Risk
If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.
Mileage / Regen Notes
SPN 3609 FMI 2 (erratic EGT Bank 1 Sensor 4) will typically inhibit active DPF regeneration due to the ACM being unable to reliably monitor post-SCR or DPF outlet temperatures. A regen lockout or derate condition may occur, and manual forced regen will likely be blocked until the fault is resolved and cleared.
System Interactions
SPN 3609 FMI 2 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.
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OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3609 FMI 2 — 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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