SPN 4364 FMI 2Aftertreatment
Aftertreatment 1 DEF Tank Temperature 2 - Erratic/Intermittent
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Overview
Fault Code
SPN 4364 / FMI 2
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: DEF tank temperature sensor 2 is mounted on or inside the DEF tank assembly, typically on the tank module alongside the DEF level and quality sensors, located under or behind the cab or on the frame rail.
Common Symptoms
- DEF quality gauge showing erratic temperature readings
- Intermittent DEF system malfunction lamp illumination
- DEF tank heater cycling erratically or not engaging
- SCR efficiency warning messages
- Inducement countdown timer active
- DEF consumption rate abnormal
Root Causes
- Faulty or degraded DEF tank temperature sensor 2 providing erratic signal Intermittent open or short in wiring harness between sensor and ACM connector Corroded or loose connector at DEF tank temperature sensor 2 DEF contamination or crystallization fouling the sensor element Poor ground reference at the sensor or ACM ground circuit ACM connector pin damage or fretting corrosion causing intermittent signal loss Failed ACM internal circuit causing erratic sensor reading interpretation
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
An erratic DEF tank temperature signal may inhibit or delay active DPF regeneration if the ACM cannot confirm DEF system readiness. The ACM may also restrict DEF dosing during regen if sensor data is deemed unreliable, potentially leading to incomplete regen cycles or elevated soot load faults.
System Interactions
SPN 4364 FMI 2 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
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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4364 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.
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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