A system and method for controlling an internal combustion engine senses differential pressure across the diesel particulate filter to monitor at least one of soot cake permeability and filter wall permeability as a function of exhaust temperature and mass flow velocity. The system and method preferably respond to both porous wall and soot cake permeabilities to determine when the soot level matches a threshold at which an operator may be warned, or at which a remedial operation may be triggered, or at which a more protective action taken by the engine control module.