A method is disclosed to test for a proper functioning of a selective catalytic reduction system of an internal combustion engine. A selective catalytic reduction system includes a catalyst disposed in an exhaust pipe of the engine, an injector disposed in the exhaust pipe upstream of the catalyst for injecting a reducing agent, a pump for providing the reducing agent from a tank to the injector via a supply conduit and a pressure sensor for measuring a pressure within the supply conduit. The engine is operated at idle speed and a functionality check of the pressure sensor is executed. The engine is operated to increase an exhaust gas temperature in the exhaust pipe upstream of the catalyst and a functionality check of the pump is executed. A functionality check of the injector is executed after the functionality check of the pump and once the exhaust gas temperature has reached a predetermined target value thereof. A functionality check of the supply conduit is executed after the functionality check of the injector. The selective catalytic reduction system is identified as functioning properly when all of the functionality checks yields a positive result or malfunctioning when any one of the functionality checks yields a negative result.