a planar sensor made of ceramic material , for measuring and detecting combustible gases and / or the oxygen content in furnaces or in the exhaust gases of internal combustion engines , is described . the sensor has an at least five - layered structure on the basis of a combination of three zirconium dioxide layers with two aluminum oxide layers , a metallic heating resistor being embedded between two aluminum oxide layers , and three zirconium dioxide layers being combined with two platinum electrodes and one further electrode made of a binary or ternary alloy of platinum with gold , nickel , copper , rhodium , ruthenium , palladium or titanium . a process for fabricating the sensor is also described . it is also possible for the sensor to be combined , according to the invention , with a suitable housing which on its outside is provided with connecting wires and plugs for transmitting the measured signals .