SPN 3720 FMI 15: Meaning and Fix
This fault indicates the Diesel Particulate Filter ash load has exceeded 100% of the target service interval. The ECM calculates ash accumulation based on oil consumption and regeneration history. In practice, this code commonly appears after a forced DPF regeneration fails to reduce ash, or when a vehicle has operated beyond its scheduled ash service without intervention. Ash loading can continue beyond 100% without immediate derate, but service is strongly recommended.
Common Symptoms
- Service Lamp On: Amber warning lamp illuminates, indicating aftertreatment ash service is due per SAE J1939.
- No Derate Yet: Engine power remains normal initially; derate may activate if ash load exceeds 150%.
- DPF Pressure High: Differential pressure sensor shows elevated readings due to ash plugging filter channels.
- Regen Frequency Up: Passive and active regenerations occur more often as soot loading increases due to reduced filter volume.
Probable Causes
- Exceeded Service Interval: Ash service not performed at recommended interval, typically 300,000-500,000 km for on-highway.
- High Oil Consumption: Excessive engine oil consumption from worn rings or turbo seals increases ash generation.
- Faulty Ash Model: ECM ash accumulation algorithm may overestimate due to incorrect oil consumption parameters.
- Regen Inefficiency: Frequent incomplete regenerations leave soot that binds with ash, accelerating filter plugging.
Advanced Technical Analysis
The ECM monitors ash load using a modeled algorithm based on engine oil consumption, regeneration events, and total fuel usage. The ash load percent is calculated as a ratio of estimated ash mass to the filter’s maximum ash capacity. When this value exceeds 100%, FMI 15 is triggered, indicating data valid but above normal range. The ECM does not immediately enforce torque reduction, but continues to track ash accumulation up to a hard limit of 150-200%.
Electrical validation of the ash load signal relies on a virtual sensor; no physical ash sensor exists. The ECM cross-checks the modeled value against differential pressure sensor readings during controlled regeneration events. If the pressure drop remains high despite successful soot oxidation, the ECM infers high ash loading. This indirect measurement can be affected by exhaust leaks or faulty pressure sensors, but the FMI 15 strictly indicates the model threshold has been crossed.
Safety fallback mechanisms for this fault are minimal below 150% ash load. Above that, the ECM may initiate a progressive torque derate of up to 25% to encourage service. The driver will notice reduced power during hill climbs or heavy loads. The derate is reversible only after a successful ash service and reset of the ash accumulation counter via diagnostic tool, following OEM procedures such as those from MAN or Mercedes-Benz.
Long-term diagnostic strategy involves verifying actual ash load via filter weighing after removal. In workshops, technicians frequently encounter this fault after replacing the ECM without resetting the ash counter, or after switching to high-ash oil. Prevention includes using low-ash CJ-4 or E6 oils and adhering to ash service intervals. Real-world example: a Deutz TCD 2013 engine showed this code after 450,000 km due to a failed turbo seal causing oil consumption of 1L/1000 km.
Step-by-Step Troubleshooting Guide
- Read Ash Load: Use diagnostic tool to view SPN 3720. Confirm value exceeds 100% and note exact percent.
- Check Oil Consumption: Review engine oil consumption history over last 100 hours. High usage (>0.5L/1000km) indicates cause.
- Perform Ash Service: Remove DPF and clean using approved thermal or pneumatic method. Weigh filter to verify actual ash mass.
- Reset Ash Counter: After service, use OEM software to reset ash accumulation parameter in ECM to 0%.
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