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Fault Code Reference

SPN 4360 FMI 18AftertreatmentMedium

SCR Intake Gas Temperature Sensor - Low

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Overview

Fault Code

SPN 4360 / FMI 18

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: SCR intake gas temperature sensor is mounted on the inlet pipe of the SCR catalyst housing, downstream of the DEF injector and decomposition tube.

Common Symptoms

  • Check engine light illuminated. MIL lamp on. Poor SCR conversion efficiency. Elevated NOx readings at tailpipe. Possible DEF dosing irregularities (under-dosing or no dosing). Fault code may be intermittent in cold weather or during cold starts. SCR inlet temperature reading abnormally low or stuck at minimum value on scan tool. May see related NOx efficiency codes (SPN 4364, SPN 3226) set alongside this fault.

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 intake temp sensor is downstream of the DPF and is primarily used for DEF dosing control, not regen management. However, if the ACM cannot accurately determine SCR bed temperature, it may suspend or reduce DEF injection to protect the catalyst from damage. Extended operation with this fault may lead to elevated soot loading or NOx efficiency faults that CAN eventually inhibit regen. Verify DPF soot load levels and check for any regen inhibit flags in live data.

System Interactions

SPN 4360 FMI 18 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 with specific bulletin numbers available — verify in OEM service portal (Cummins QuickServe, Detroit DemandSide, PACCAR ESA, Navistar OnCommand) for any applicable campaigns related to SCR temperature sensor wiring harness chafing or sensor updates for your specific engine serial number and build date.

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 4360 FMI 18 — 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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