SPN 3720 FMI 16: Meaning and Fix
This fault indicates that the diesel particulate filter ash load has exceeded 100%, the target service threshold. The ECM calculates ash load based on oil consumption model and regeneration history. In practice, this code commonly appears after prolonged operation with high oil consumption or when ash service has been deferred beyond the recommended interval, causing the ECM to flag a moderately severe over-range condition.
Common Symptoms
- Derated Engine Power: ECM reduces torque by up to 25% to protect the DPF from excessive backpressure.
- Frequent Regeneration: Passive and active regenerations occur more often due to reduced soot capacity from ash buildup.
- Exhaust Backpressure High: Measured delta pressure across DPF rises above 50 kPa at rated load per SAE J1939.
- Service Lamp Illuminated: Amber warning lamp activates on dashboard, indicating ash service is due immediately.
Probable Causes
- Ash Service Overdue: DPF not cleaned per manufacturer interval, typically every 3000-5000 engine hours.
- Excessive Oil Consumption: Worn piston rings or turbo seals increase ash generation from lube oil additives.
- Incorrect Oil Specification: Use of high-ash oil (e.g., CJ-4 vs CK-4) accelerates ash accumulation in the DPF.
- Faulty Ash Model Calibration: ECM software incorrectly estimates ash load due to corrupted learned values or sensor drift.
Advanced Technical Analysis
The ECM calculates ash load percent using a predictive model based on oil consumption rate, regeneration events, and engine hours. Per Bosch MD1 and MAN D26 factory documentation, the model assumes a fixed ash content per liter of oil consumed (approx. 1.5 g/L for low-ash oils). When calculated ash exceeds 100%, FMI 16 is set. Actual ash load can be verified by weighing the DPF before and after cleaning; discrepancies >10% indicate model inaccuracy.
Electrical signal integrity is not directly involved for this SPN, as it is a calculated parameter. However, the ECM monitors the differential pressure sensor (SPN 3610) and exhaust temperature sensors (SPN 3251, 3252) for plausibility. If sensor readings deviate by more than 15% from expected values during regeneration, the ash model may be incorrectly updated, leading to premature FMI 16 activation. Per Deutz TCD 2015 manuals, sensor drift is a common confounding factor.
When FMI 16 is active, the ECM initiates a multi-level derate strategy. According to Mercedes-Benz OM471 factory documentation, the first stage reduces maximum torque by 10% at 105% ash load, increasing to 25% at 120%. The derate is applied gradually to avoid sudden power loss. The ECM also inhibits automatic regeneration if backpressure exceeds 60 kPa, forcing the operator to perform stationary regeneration or service the DPF.
Long-term prevention requires tracking oil consumption trends and adhering to ash service intervals. In real-world workshops, this fault often appears on refuse trucks with high idle hours. Technicians should inspect turbo seals and piston rings if oil consumption exceeds 0.5 L/100 h. After cleaning the DPF, the ash load model must be reset via diagnostic tool (e.g., DAVIE or CAT ET) to clear the fault and restore full power.
Step-by-Step Troubleshooting Guide
- Verify Ash Load Value: Use diagnostic tool to read SPN 3720. If >100%, proceed with ash service.
- Inspect Oil Consumption: Check engine oil level trend over 100 hours; >0.5 L/100 h indicates mechanical wear.
- Perform DPF Cleaning: Remove DPF and clean using thermal or pneumatic method per OEM specifications.
- Reset Ash Model: After cleaning, reset ash load learned values in ECM using factory scan tool.