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Various proposals for emissions cleanup systems have shown the desirability of regulating engine air-fuel ratio within precise limits. For this purpose a prototype exhaust sensor has been investigated. The sensor is a ceramic device, made of stabilized zirconia, which operates via electrochemical principles. It is placed directly in the exhaust stream and generates a voltage signal which is an approximate indication of engine air-fuel ratio. Several sensors have been installed in situ on engines operated under controlled dynamometer conditions. Fundamental response characteristics of the sensors have been determined. The results of this investigation, together with descriptions of the construction and installation configuration of the prototype sensor, are discussed.