Simple tests were performed to investigate the operating characteristics of zirconia galvanic cells (lambda sensors) in automotive closed loop “three-way” emission control systems. Commercially available cells were exposed to typical gaseous components of exhaust gas mixtures. The voltages generated by the cells were at their maximum values when hydrogen, and, in some instances, carbon monoxide, was available for reaction with atmospheric oxygen that migrated through the cells' ceramic thimbles in ionic form. This dependence of galvanic activity on the availability of these particular reducing agents indicated that the cells were voltaic devices which operated as oxidation/reduction reaction cells, rather than simple oxygen concentration cells. Such operation explains why a cell that is used as a lambda sensor in a closed-loop control system exhibits a sixfold or greater decrease in voltage output when the exhaust gas composition changes from a slightly rich condition (lambda = 0.995) to a slightly lean condition (lambda = 1.005). It also explains why the voltage of a cell that is located downstream of a properly operating catalyst normally remains at a low level as the air/fuel ratio oscillates around the stoichiometric value but increases to a high level when ignition misfire occurs at a rate that exceeds a certain value.