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Fault Code Reference

SPN 4363 FMI 0AftertreatmentMedium

SCR Outlet Temperature - High

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Overview

Fault Code

SPN 4363 / FMI 0

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: SCR outlet temperature sensor is mounted on the exhaust pipe downstream of the SCR catalyst, typically before or near the exhaust outlet on the aftertreatment assembly.

Common Symptoms

  • High exhaust temperature reading after SCR catalyst on scan tool. MIL lamp illuminated. Possible DEF quality warning light. Exhaust smells abnormal (ammonia smell if over-dosing). Reduced fuel economy. Potential white smoke from exhaust during certain operating conditions. Driver may report derate warning on dash. SCR efficiency codes may also be present.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault typically does not inhibit active DPF regeneration directly, as it is an SCR system fault rather than a DPF-related fault. However, if the ECM/ACM sees abnormally high SCR outlet temps, it may limit or delay regen events to prevent perceived overheating of aftertreatment components. Monitor for regen inhibit flags in software. Parked regen should still be available unless additional faults are present.

System Interactions

SPN 4363 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
TSB Reference: No confirmed TSBs — verify in OEM service portal. Check for bulletins related to SCR temperature sensor harness routing and heat shield updates. Cummins, Detroit, and Paccar have issued periodic updates for aftertreatment sensor wiring chafing issues — search by SPN 4363 in OEM system.

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.

Deep-dive diagnostic page: SPN 4363 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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