Composite sensor

The present invention relates to a gas sensor, and particularly to a combined gas sensor for detecting the concentration of acid gases such as nitrogen oxides, etc. in exhaust gases and oxygen concentration in a gas atmosphere to be detected. A combined gas sensor of the invention is equipped with a second oxygen sensing electrode active only to oxygen, which is disposed on a solid electrolyte substrate such that it is directly exposed to detection gases. Sensor disorder is detected by measuring an electromotive force (signal output ) generated between a reference electrode and the second oxygen sensing electrode as an oxygen concentration in an exhaust gas atmosphere, and comparing and judging the detected result with a signal output corresponding to an electromotive force between a first oxygen sensing electrode and the sit reference electrode , or with a signal output of an oxygen pump current.