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Fault Code Reference

SPN 4363 FMI 4AftertreatmentMedium

SCR Outlet Temperature Sensor - Voltage Low

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Overview

Fault Code

SPN 4363 / FMI 4

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: The SCR outlet temperature sensor is located in the exhaust system downstream of the SCR catalyst, typically before the tailpipe.

Common Symptoms

  • Check engine light illuminated. MIL lamp on. SCR outlet temperature reading extremely low, zero, or displaying dashes on scan tool. Possible reduced fuel economy. DEF dosing may be suspended or erratic. Frozen or inaccurate aftertreatment temperature PIDs. May see related SCR efficiency codes if DEF injection is affected.

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 the SCR outlet temp sensor is downstream of the DPF and primarily monitors SCR catalyst performance. However, if the ACM cannot verify proper SCR temperatures, it may suspend or limit DEF injection to protect the catalyst. Regens should proceed normally, but monitor for secondary codes related to SCR efficiency or DEF dosing if this fault persists.

System Interactions

SPN 4363 FMI 4 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 OEM-specific bulletins related to aftertreatment harness routing, connector sealing updates, or sensor kit revisions. Cummins, Detroit, and Paccar have issued various harness rework advisories for exhaust-mounted sensor wiring — search by SPN 4363 in respective service systems.

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 4 — 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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