Gas concentration measuring apparatus with failure monitor

A gas concentration measuring apparatus comprising a gas concentration sensor equipped with a sensor element which is made of a solid electrolyte body and a pair of electrodes affixed to the solid electrolyte body, a sensor control circuit connected to the electrodes of the sensor element through a positive and a negative terminal, when a gas concentration measuring mode is entered, said sensor control circuit applying a voltage across the electrodes of the sensor element within a given applied-voltage control range to produce a flow of electrical current through the sensor element and sampling the electrical current to output a sensor current signal as indicating a concentration of a gas to be measured, said sensor control circuit also sampling voltages appearing at the positive and negative terminals, and a failure monitor working to monitor a failure in said gas concentration sensor, when the voltages at the positive and negative terminals, as sampled by said sensor control circuit, are identical with each other, one of the voltages at the positive and negative terminals lies within the applied-voltage control range, and the sensor current signal has an unusual value, said failure monitor determining that an electrical short has occurred between the positive and negative terminals.