METHOD FOR DIAGNOSING UNUSUAL CONDITIONS OF GAS CONCENTRATION DETECTING UNIT, AND APPARATUS FOR DIAGNOSING UNUSUAL CONDITIONS OF GAS CONCENTRATION DETECTING UNIT

To provide a malfunction diagnosing method and apparatus capable of reliably determining a malfunction even where the malfunction occurs in a common wire connecting a gas sensor and a control circuit for driving it. In a gas concentration measurement unit 1 having a sensor control circuit 50 which is connected to a sensor element 10 including an oxygen pump cell 14 and an oxygen concentration detection cell 24 and controls and drives this sensor element 10, a variation state of a terminal voltage at one connecting point (specifically, a Vs+ terminal) other than connecting points between electrodes facing a measurement chamber of each cell 14 and 24 and a common wire 24 is detected by a malfunction detection circuit 53 and an arithmetic processor 54. Based on this variation state of the terminal voltage, diagnosis of malfunction in the common wire 42 is executed. When the common wire 42 is broken, the terminal voltage at the Vs+ terminal varies with oscillations. Therefore, the malfunction detection circuit 53 and the arithmetic processor 54 detect this voltage variation with oscillations to output a malfunction determination signal to a CPU.