Abstract: A thin film mutilayered air-fuel ratio sensor having a sensor element including a first electrode formed on a porous substrate, a first solid electrolyte and a third electrode formed on the first electrode in this order, in such a manner that the first electrode is completely covered by the first solid electrolyte inclusive of its surroundings, and that the first solid electrolyte is completely covered by the third electrode inclusive of its surroundings, a second solid electrolyte and a fourth electrode and formed on the third electrode in this order, in such a manner that the peripheral portion of the third electrode is left uncovered by the second solid electrolyte and the fourth electrode and thereby left exposed. The first electrode, the third electrode, and the fourth electrode are made of a gas-permeable porous platinum, and the first solid electrolyte and the second solid electrolyte are made of a gas-permeable dense solid electrolyte having oxygen ion conductivity.

Abstract: An oxygen sensor for sensing the oxygen content of an exhaust gas from an automotive engine and a process for producing the same, which sensor is less sensitive to lead poisoning than conventional sensors. In the inventive sensor, a first platinum electrode is formed on the side of a solid electrolyte body exposed to the gas to be detected, and a second electrode is formed on the other side of the electrolyte body. The first electrode comprises a platinum layer and a layer of at least one of rhodium and palladium. Preferably, the layer of rhodium or palladium has a thickness of 10 to 5,000 .ANG..