A gas sensor formed as an electrochemical cell having a small number of components and being inexpensive to manufacture comprises electrodes (22, 24) formed as porous planar elements on a substrate (20), the substrate being porous to permit permeation of gas to the electrodes from the exterior of the gas sensor, external electrode terminals (10) mounted in the periphery of a housing (4) wherein in the region of the terminal ends, the electrodes, substrate and housing are subject to a heat and pressure welding operation in order to hold the terminals and electrodes in electrical connection with one another while blocking the porosity of the electrodes in order to prevent electrolyte permeating through the electrodes to the electrical connection.