This tap regulates the flow as well as switching a 12v pump. The microswitch on my previous one failed after 2 years, with dirty contacts. Once you know, these can be cleaned without removing the tap. Unclip the knob top from the body by forcing upward with a screwdriver between the water outlet and knob. There is a hexagon topped screw assembly inside the knob. Note position and remove. Microswitch can be removed by sliding a small scredwiver or knife blade down the side of the plastic block holding the contacts and springs and lever out - take care not to bend the contacts. Clean contacts with emery cloth and reassemble.