Diagnostic Overview
Detroit SPN 3720 commonly points to DPF ash load or aftertreatment service life monitoring. Unlike soot, ash does not burn out during normal regeneration, so a high ash-load condition can require filter cleaning or replacement. This type of code often appears after high mileage, repeated regens, or long service intervals.
Likely Causes
DPF ash accumulation, high mileage service interval, oil consumption, excessive engine blow-by, incorrect oil, DPF service history not reset, or a filter that has already reached its ash capacity.
What To Check First
Check DPF ash/soot data, review service history, inspect for oil consumption or crankcase pressure issues, and confirm whether the DPF was cleaned or replaced and whether the service reset procedure was completed. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.