Method of operating an aftertreatment system of an internal combustion engine

The method of operating an aftertreatment system of an internal combustion engine, having the step of: monitoring an oxygen concentration upstream of an aftertreatment device; monitoring a fuel rail pressure value; calculating a combustion parameter by multiplying an oxygen concentration value by a fuel rail pressure value 640; comparing the combustion parameter with a reference value thereof 650; and, if the combustion parameter is greater than the reference value, activate a protection procedure of the aftertreatment device. This allows for a better identification of the soot or ash type produced by the engine. This helps to avoid unwanted thermal events which can lead of excessive aging of the material of the filter. The protection procedure maybe be a regeneration of the aftertreatment device or a change in the temperature set-point for initiating an aftertreatment device regeneration, the aftertreatment device can be a diesel particulate filter (DPF).