What are the symptoms of SPN 3246 FMI 9?

Check engine light illuminated. Engine derate or power reduction. DPF regeneration may not complete properly. Erratic exhaust temperature readings displayed. Aftertreatment system warnings on dash. Forced parked regeneration required more frequently

What causes SPN 3246 FMI 9?

Loss of CAN bus communication between ACM and sensor module causing missed update rate Damaged, corroded, or loose connector at DPF Exhaust Temperature Sensor 2 Intermittent open or short in the wiring harness between sensor and ACM Failed or out-of-range DPF Exhaust Temperature Sensor 2 (downstream of DPF) ACM software or calibration issue causing incorrect data rate interpretation Faulty ACM module failing to properly receive sensor data Terminating resistor fault on CAN network affecting data transmission

How do I diagnose SPN 3246 FMI 9?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 3246 FMI 9 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 3246 FMI 9AftertreatmentInducement Risk

DPF Exhaust Temperature Sensor 2 - Abnormal Update Rate

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Overview

Fault Code

SPN 3246 / FMI 9

Common Symptoms

Check engine light illuminated
Engine derate or power reduction
DPF regeneration may not complete properly
Erratic exhaust temperature readings displayed
Aftertreatment system warnings on dash
Forced parked regeneration required more frequently

Root Causes

Loss of CAN bus communication between ACM and sensor module causing missed update rate Damaged, corroded, or loose connector at DPF Exhaust Temperature Sensor 2 Intermittent open or short in the wiring harness between sensor and ACM Failed or out-of-range DPF Exhaust Temperature Sensor 2 (downstream of DPF) ACM software or calibration issue causing incorrect data rate interpretation Faulty ACM module failing to properly receive sensor data Terminating resistor fault on CAN network affecting data transmission

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

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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 3246 FMI 9 will typically inhibit active DPF regeneration because the ACM cannot reliably monitor exhaust temperatures during regen. Parked and forced regen may be blocked. Continued inhibition can lead to DPF over-loading and potential derates.

System Interactions

SPN 3246 FMI 9 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

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3364-2DEF Quality Erratic SPN 3251-0DPF Differential Pressure High SPN 3719-16DPF Soot Load High

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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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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