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Fault Code Reference

SPN 3720 FMI 0AftertreatmentHighInducement Risk

DPF Ash Load - High

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Overview

Fault Code

SPN 3720 / FMI 0

Category

Aftertreatment

Severity

High

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: SPN 3720 is a calculated/modeled value derived by the Aftertreatment Control Module (ACM) or Engine Control Module (ECM) — it is not a direct sensor reading from a single physical component. The ash load value is computed from a combination of DPF differential pressure sensor data, exhaust mass flow, and accumulated service history stored in the ECM. Physical components involved: (1) DPF (Diesel Particulate Filter) — mounted in the aftertreatment assembly, typically located under the vehicle frame, behind the cab, or on the driver-side frame rail depending on the platform and chassis. On many Class 8 trucks, the aftertreatment system (DPF + DOC + SCR) is housed in a single canister or modular assembly mounted vertically or horizontally on the frame. (2) DPF Differential Pressure Sensor — typically mounted on or near the DPF housing, with sensing tubes connected upstream and downstream of the DPF substrate. Exact location varies significantly by OEM and chassis configuration. Verify exact DPF and sensor location in the OEM chassis/engine service manual for the specific vehicle. Based on: General Class 8 aftertreatment system layout documentation, Cummins/Detroit/PACCAR aftertreatment service guides.

Common Symptoms

  • Extended or frequent regeneration cycles with limited effectiveness, progressive power derate (5 MPH increments down to 5 MPH in severe cases), elevated exhaust backpressure readings, reduced fuel economy, MIL/CEL illuminated, possible engine shutdown warning in advanced inducement stages, driver complaints of sluggish performance especially under load, DPF differential pressure readings remain high even after successful regen completion

Root Causes

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

Active and parked regenerations will still occur and complete normally, but they CANNOT remove ash. Ash is non-combustible (metalite oxides from engine oil additives) and remains in the DPF regardless of regen temperature or duration. Regens will continue to burn off soot, but ash load percentage will not decrease. Technicians should not attempt repeated regens to clear this code — physical DPF ash cleaning or replacement is the only solution.

System Interactions

SPN 3720 FMI 0 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: Ash service TSBs (Cummins/Detroit/PACCAR)

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 0 — 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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