Abstract: A sensor for determining at least one of gas constituents and gas concentrations in a gas mixture, such as CO, NO.sub.x and HC, in exhaust gas from an internal combustion engine, includes a semiconductor gas sensor having a gas sensitive region in communication with the gas mixture; a pumping cell including a solid electrolyte carrier and pumping electrodes disposed on the solid electrolyte carrier, the pumping cell being positioned in communication with the semiconductor gas sensor for effecting oxygen transfer to the semiconductor gas sensor on the side thereof opposite the gas mixture by pumping oxygen thereto to provide an O.sub.

Abstract: A method for monitoring a catalytic converter for vehicles having Otto engines includes monitoring the composition of exhaust gases leaving a catalytic converter. An apparatus for monitoring the catalytic converter includes a sensor for hydrogen and/or carbon monoxide, and a sensor for oxygen.