SPN 3720 FMI 3AftertreatmentMedium
DPF Ash Load Sensor - Voltage High
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Overview
Fault Code
SPN 3720 / FMI 3
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: The DPF ash load sensor is typically integrated into the DPF differential pressure sensor assembly mounted on the DPF housing.
Common Symptoms
- Check engine light illuminated. Amber warning lamp on dash. Possible inaccurate ash load readings in diagnostic software. DPF maintenance indicator may show incorrect service intervals. Technician may notice ash load percentage stuck at maximum or reading abnormally high. No immediate driveability symptoms in most cases, but long-term DPF service scheduling will be affected.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does not inhibit active regeneration. The ash load sensor is used primarily for DPF service life tracking and maintenance scheduling, not for real-time regen control. Soot load sensors and differential pressure sensors control regen timing. However, if the ECM calculates ash load has reached maximum based on faulty high voltage signal, it may flag the DPF for service or replacement sooner than necessary. Active and parked regens should still function normally.
System Interactions
SPN 3720 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 TSBs with specific numbers available — verify in OEM service portal for your specific engine platform (Cummins, Detroit, Paccar, etc.). Some OEMs have released service bulletins addressing aftertreatment sensor harness routing and connector sealing issues. Check for updated harness repair kits or sensor revisions.
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 3720 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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