Method and system of testing the proper functioning of a catalyzed particulate filter of an internal combustion engine

A system and method of testing a catalysed particulate filter 290 such as a CDPF included in an after-treatment system 270 of an internal combustion engine. A regeneration of the filter is performed using an ECU during which the amount of particulate trapped in the filter is determined and when low enough values of an oxygen concentration in an exhaust gas are measured upstream and downstream of the filter using oxygen sensors 291/292 such as lambda or nitrogen oxide sensors. The difference between the values is calculated possibly by the ECU integrating the difference over a predetermined time and a malfunctioning filter identified if the difference is below a predetermined threshold value whereupon the driver may be warned. The predetermined threshold may be determined by a volumetric flow rate of the exhaust gas, engine speed and engine torque. The after-treatment system may also include an EGR duct and diesel oxidation catalysts 285 (DOC), NOx traps or selective catalytic reduction systems.