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This paper describes the sensing principle of a new air-fuel ratio sensor, which has the ability to detect air-fuel ratios in rich, stoichiometric and lean ranges. The sensing part is composed of a gas diffusion layer and a zirconia solid electrolyte with a pair of electrodes which function as an anode and a cathode. The anode and the cathode electrodes are exposed to the atmosphere and the exhaust gas, respectively. To obtain the bidirectional pumping current between the two electrodes, the potential of the cathode is held to a constant value higher than the electronic circuit ground. The electromotive force induced between the two electrodes is forcibly controlled to a constant value by the electronic circuit. In this composition, three ranges of air-fuel ratio can be detected by the amount of pumping current.