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Fault Code Reference

SPN 3720 FMI 4AftertreatmentMedium

DPF Ash Load Sensor - Voltage Low

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Overview

Fault Code

SPN 3720 / FMI 4

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 diesel particulate filter housing in the exhaust system.

Common Symptoms

  • Check engine light illuminated. MIL lamp on. Possible yellow engine warning lamp. Inaccurate ash load readings in scan tool (may show 0% or unusually low). DPF maintenance intervals may not calculate correctly. Possible false high soot load readings as ECM compensates. No immediate driveability issues in most cases.

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 primarily used for DPF service interval calculations, not real-time regen control. The ECM will continue to use differential pressure and soot load calculations to manage regens. However, if the ECM cannot accurately estimate ash accumulation, it may default to conservative ash load assumptions which could affect long-term DPF cleaning recommendations. Active and passive regen should function normally with this fault present.

System Interactions

SPN 3720 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. Some OEMs have issued bulletins for aftertreatment sensor harness routing and chafing issues. Check for chassis-specific service letters regarding DPF sensor connector sealing and water intrusion.

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 3720 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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