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Fault Code Reference

SPN 4363 FMI 2Medium

SCR Outlet Temperature - Data Erratic

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Overview

Fault Code

SPN 4363 / FMI 2

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: SCR outlet temperature sensor is mounted in the exhaust outlet pipe immediately downstream of the SCR catalyst brick, typically on the rear or underside of the aftertreatment assembly.

Common Symptoms

  • SCR outlet temperature reading erratic or fluctuating abnormally
  • Active fault code logged for SPN 4363 FMI 2
  • Aftertreatment system inhibiting or interrupting DEF dosing
  • DPF regeneration inhibited or interrupted
  • SCR efficiency faults triggered secondary to unstable temp signal
  • Intermittent or stuck temperature reading on scan tool data

Root Causes

  • Damaged, corroded, or loose connector at SCR outlet temperature sensor
  • Failed or failing SCR outlet temperature sensor (thermistor degradation)
  • Chafed, shorted, or open wiring harness between sensor and ACM/ECM
  • Moisture or contamination intrusion into sensor connector
  • Poor ground reference at sensor or aftertreatment control module
  • Aftertreatment Control Module (ACM) internal fault causing erratic signal interpretation
  • Exhaust soot or chemical contamination on sensor tip affecting thermal response

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Derate Behavior

Mileage / Regen Notes

Erratic SCR outlet temperature signal may inhibit active DPF regeneration initiation or cause an in-progress regen to be aborted, as the ACM relies on this signal to confirm safe exhaust temperatures downstream of the SCR catalyst.

System Interactions

SPN 4363 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: SCR 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 4363 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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