Zirconia galvanic cells of the formare installed in the exhaust pipe of automobile engines to provide a voltage signal indicative of engine air‐fuel ratio. To account for deviation of experimental sensor behavior from predicted ideal performance, a physical model of nonideal sensors is derived. Nonideal behavior is accounted for by simultaneous presence of two electrochemical reactions occurring at the anode (exhaust gas) electrode; one reaction involves oxygen and the other involves carbon monoxide. Correlation of calculated sensor characteristics with experimental results shows that nonideal sensor electrodes possess surface adsorption properties and catalytic activities which are many orders of magnitude less than ideal equilibrium values. The present theory indicates that undesired temperature‐dependent shifting of sensor voltage curves with respect to engine air‐fuel ratio is caused by competition between and gases for triplepoint adsorption sites on the anode electrode.