SPN 3720 FMI 16 — Aftertreatment DPF Ash Load – Above Normal, Moderately Severe — Detroit DD15

SPN 3720 FMI 16 on Detroit DD15 (2010–present) indicates the calculated DPF ash load is above normal, moderately severe. Ash accumulates over time and does not…

Updated October 15, 2026

What it means

The calculated DPF ash load is above normal, moderately severe. On Detroit DD15, ash does not burn off in regeneration; the DPF typically needs removal and cleaning or replacement at its ash service interval.¹

Most likely causes

1. Ash accumulation has reached the service threshold; the DPF requires cleaning or replacement. — Ash is incombustible residue from oil additives and normal engine wear. Over the ash service interval the filter becomes progressively restricted. The platform note is that ash does not burn off in regeneration and the DPF needs to be removed and cleaned or replaced at its ash service interval.¹
2. DPF differential pressure is elevated due to ash loading. — A loaded DPF raises differential pressure. SPN 3251 FMI 0 is described as DPF differential pressure far above normal, typical of a soot- or ash-loaded DPF, or plugged/damaged pressure sensor tubes reading falsely high; a parked regeneration may clear soot, ash needs filter cleaning.²
3. Pressure sensor or sensing lines are compromised, causing an inaccurate ash load calculation. — Erratic or implausible DPF outlet pressure can skew load calculations. SPN 3610 FMI 2 is DPF outlet pressure erratic, intermittent or incorrect, typical of a plugged or cracked sensor line, a failing pressure sensor or wiring fault.³
4. Repeated interrupted or failed regenerations have allowed soot to accumulate and mask as increased restriction. — Soot load faults are related. SPN 3719 FMI 16 is DPF soot load above normal, moderately severe, typical of interrupted or failed regenerations, short-trip duty cycles, or engine-side soot contributors.⁴

How to diagnose

1. Verify the code is active and review ACM/MCM data for DPF ash load %, differential pressure, and recent regeneration history. Compare ash load % to the Detroit ash service interval for the specific EPA generation.
2. Inspect DPF differential pressure at idle and at rated speed/load. Elevated pressure consistent with ash loading supports service need; compare to SPN 3251 behavior.
3. Inspect DPF differential pressure sensor tubes and connections for plugging, moisture, cracks, or exhaust leaks, and check sensor plausibility. Erratic outlet pressure faults point to sensor/line issues.
4. Review regeneration history and duty cycle. Frequent interrupted regenerations or short-trip operation can increase soot loading and affect load calculations.
5. Check for accompanying aftertreatment faults such as NOx sensor anomalies that can affect ACM calculations. Intake NOx below normal can indicate a failing inlet NOx sensor, wiring or an exhaust leak near the sensor.
6. If ash load remains above normal after confirming sensor integrity and completing a successful parked regeneration to clear soot, plan DPF removal for ash cleaning or replacement per Detroit service interval. Ash does not burn off in regeneration.

Common mistake to avoid

Cleaning or replacing the DPF before checking its pressure sensor and tubes. Cracked, melted, plugged or wet sensor tubes, or a failing sensor, can set DPF codes while the filter itself is fine. They are cheap to check; clear and verify them before touching the filter. Source: FleetRabbit — P2002 DPF efficiency code causes and fixes.

Parts and tools you may need

• Diagnostic software capable of reading Detroit MCM/ACM/CPC J1939 SPN/FMI data
• Digital manometer or scan tool DPF differential pressure readout
• DPF pressure sensor and tube inspection kit
• Exhaust backpressure test equipment
• Parked regeneration initiation capability via service tool

Tools you’ll need

• diagnostic scan tool — Reads SPN 3720 FMI 16 and ACM/MCM data for DPF ash load %, differential pressure, and regeneration history to confirm the ash service threshold.
• digital multimeter — Checks DPF differential pressure sensor supply, ground, and signal voltages for plausibility and wiring faults that could skew ash load calculations.
• manometer — Measures actual DPF differential pressure at idle and rated speed/load to verify elevated restriction consistent with ash loading versus sensor error.
• borescope — Inspects DPF differential pressure sensor tubes and connections for plugging, moisture, cracks, or exhaust leaks without disassembly.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If ash load remains high after sensor verification and a successful parked regeneration, have a qualified Detroit technician remove and service or replace the DPF per the ash service interval. Do not attempt high-pressure fuel system service. If operator inducement or derate appears, address the root aftertreatment fault promptly.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources