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Fault Code Reference

SPN 3246 FMI 2Medium

DPF Outlet Temperature - Data Erratic

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Overview

Fault Code

SPN 3246 / FMI 2

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: DPF outlet temperature sensor is mounted in the exhaust bung on the downstream (outlet) end of the diesel particulate filter, typically along the frame rail or under the cab.

Common Symptoms

  • DPF outlet temperature reading is erratic or fluctuating abnormally
  • Intermittent or stuck temperature sensor readings in diagnostic data
  • Active or passive DPF regeneration inhibited or interrupted
  • MIL or amber warning lamp illuminated
  • Possible engine derate condition
  • Regen events failing to complete or not initiating

Root Causes

  • Faulty or degraded DPF outlet temperature sensor (thermistor failure)
  • Damaged, corroded, or water-intrusion-affected wiring harness connector at sensor
  • Chafed or shorted wiring between sensor and aftertreatment control module
  • Poor ground reference at sensor or control module
  • Sensor contaminated with soot, oil, or moisture causing erratic signal
  • Failed or intermittent aftertreatment control module (ACM/DCM)
  • Loose or improperly seated sensor in bung causing thermal instability

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Derate Behavior

Mileage / Regen Notes

DPF passive and active regeneration is inhibited or interrupted when outlet temperature data is erratic; the aftertreatment system cannot accurately monitor DPF exit temperature, preventing safe regen completion and potentially triggering regen lockout.

System Interactions

SPN 3246 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: DPF temp TSB

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.

Deep-dive diagnostic page: SPN 3246 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.

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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