The invention refers to a plate heat exchanger, which includes a sensor device and a plate package of heat transfer plates (1). The heat transfer plates form between the plates (1) first passages (3) for a first fluid and second passages (4) for a second fluid. The sensor device includes a space (21), which is closed to the first passages (3) and the second passages (4). The closed space (21) is arranged to contain a medium, which can be influenced by the temperature of at least one of the fluids, and to be connectable to a device (14) for sensing a pressure change of the medium in the closed space (21). The closed space (21) is at least partly defined by at least one of the plates (1).