SPN 4363 FMI 3AftertreatmentMedium
SCR Outlet Temperature Sensor - Voltage High
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Overview
Fault Code
SPN 4363 / FMI 3
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 immediately downstream of the SCR catalyst brick, typically on the outlet end cap of the aftertreatment housing.
Common Symptoms
- Check engine light illuminated. MIL lamp on. SCR outlet temperature reading pegged high or showing implausible value (often 3000°F+ or max sensor range). Possible reduced fuel economy. DEF dosing may be reduced or suspended. Aftertreatment warning lamp may illuminate. Scan tool shows voltage above normal operating range (typically above 4.8V). Possible ammonia slip or elevated NOx readings due to incorrect DEF dosing calculations.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does NOT inhibit active regeneration directly. The SCR outlet temp sensor is downstream of DPF and primarily used for SCR efficiency monitoring and DEF dosing calculations, not regen control. However, if the ACM cannot determine accurate exhaust temps, it may default to conservative DEF dosing strategies. Passive and active regens generally continue but DEF injection may be suspended or limited until the fault is repaired, which can lead to elevated NOx.
System Interactions
SPN 4363 FMI 3 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 universal TSBs — verify in OEM service portal. Cummins ISX/ISB platforms may have service letters related to aftertreatment harness routing and heat shield installation. Detroit DD13/DD15 has known harness chafing areas near SCR. Check OEM portal for model-year specific campaigns.
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
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4363 FMI 3 — 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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