A diaphragm pump includes a pressure sensor disposed in the housing thereof, and which is used as an expansion tank for protecting the pressure sensor. To avoid excess pressure on the pressure sensor, the pressure sensor membrane is directly disposed in the pump head, i.e., above the pump membrane of the diaphragm pump, such that the increase in volume of the reducing agent during the freezing of the reducing agent inside the diaphragm pump is detected by deflection of the pump membrane against the force of a spring, and the pressure sensor membrane is protected from damage.