SPN 3720 FMI 20: Meaning and Fix
This fault indicates that the ash load percent signal from the DPF has drifted above the plausible high threshold, often due to sensor drift or incorrect ash modeling. Technicians commonly see this after a forced regeneration where ash accumulation was not reset, or following an ECM replacement with mismatched calibration. The ECM compares calculated ash load against a model; a persistent high drift suggests the differential pressure sensor or soot model parameters are out of range.
Common Symptoms
- Service lamp on: Amber warning lamp illuminates on dash, indicating aftertreatment service required due to high ash load.
- DPF indicator active: DPF regeneration inhibit lamp may flash, preventing active regen until ash load is addressed.
- Torque reduction: Engine torque may be derated by up to 25% to protect DPF from further ash buildup.
- Frequent regen requests: Vehicle requests passive or active regenerations more often as ash load model skews high.
Probable Causes
- Sensor drift: Differential pressure sensor output shifted high due to contamination or age, misreporting ash load.
- ECM calibration error: Incorrect ash load model parameters after ECM flash or replacement cause false high readings.
- Ash service not reset: After ash cleaning, the ash load parameter was not reset via diagnostic tool, retaining prior high value.
- Wiring fault: Signal wire short to battery voltage or sensor ground offset causes data drift above normal range.
Advanced Technical Analysis
The ECM monitors the ash load percent via a model that integrates soot and ash accumulation based on exhaust flow, differential pressure, and regen events. When the calculated value exceeds 100% and the sensor input drifts high, FMI 20 triggers. The ECM applies a debounce timer of 10 seconds to confirm persistent drift before setting the fault, preventing transient noise from causing false positives.
Electrical analysis shows that a drift high often originates from the differential pressure sensor’s 5V reference line or signal line. If the signal voltage exceeds 4.75V (indicating >100% ash load), the ECM interprets this as data drifted high. A short to VBATT or a failing sensor with internal leakage can cause this. Proper grounding and shielded wiring are critical to avoid induced voltages.
When FMI 20 is active, the ECM may inhibit automatic regenerations and limit engine torque to 75% of rated power. This fallback protects the DPF from over-pressurization and potential substrate damage. The derate is gradual, increasing over 30 minutes if the fault persists. Some OEMs allow a temporary override for emergency vehicle operation, but only with verified low ash load.
Long-term prevention requires resetting ash load parameters after every manual ash cleaning using a factory scan tool. In the field, technicians often encounter this fault on trucks that have undergone multiple forced regenerations without ash service. A real-world example is a 2020 MAN TGS where the ash load read 112% after a DPF replacement; resetting the ash model and recalibrating the pressure sensor cleared the fault permanently.
Step-by-Step Troubleshooting Guide
- Scan tool check: Read live ash load percent; compare to actual service history. If >100% and not serviced, perform ash cleaning.
- Sensor test: Measure differential pressure sensor voltage at idle and 2500 rpm; should be 0.5-4.5V. Drift above 4.75V indicates fault.
- Wiring inspection: Check signal and reference wires for shorts to power or ground. Repair any damaged shielding or loose connectors.
- Reset procedure: After ash service, use diagnostic tool to reset ash load parameter to 0%. Verify value drops below 5% to clear fault.