Filter diagnostics method and arrangement

An operational status of apparatus for adsorbing and desorbing pollutants is monitored by adsorbing pollutants in filter in adsorbing mode, desorbing pollutants to engine during desorption mode, and generating error signal if amount desorbed differs from adsorbed amount. An independent claim is also included for an arrangement for monitoring operational status of the apparatus for adsorbing and desorbing pollutants. The apparatus includes a filter (5) for adsorbing pollutants causing gas to flow through the filter during adsorption cycle; mechanism for causing gas to flow through the filter into the engine during desorption cycle; source of heat for heating the filter; controllable valves for controlling direction of gas flow during different cycles; and central processing unit for controlling the valves in response to input signals from sensors and for monitoring function of the sensors. There is a sensor for measuring pollution concentration in the gas and is positioned upstream of the filter. There is a sensor for measuring pollution concentration in the desorbed gas and is positioned downstream of the filter. The central processing unit is arranged to use signals from the pollution sensor for monitoring adsorption of pollutants in the filter, to monitor desorption of pollutants from the filter, and to generate the error signal.