Patent ID: 7850840

Claim:
A method for diagnosing a malfunction of a gas concentration detecting unit comprising: a gas sensor including an oxygen pump cell including a first solid electrolytic layer and a pair of electrodes interposing the first solid electrolytic layer therebetween, an oxygen concentration detection cell including a second solid electrolytic layer and a pair of electrodes interposing the second solid electrolytic layer therebetween, and a measurement chamber in which a gas to be measured is introduced and one of the electrodes of each cell is placed therein, and a control circuit electrically connected to the respective electrodes of the cells, the control circuit controlling a current flowing through the oxygen pump cell so that a voltage outputted from the oxygen concentration detection cell will be a pre-set value, thereby controlling an oxygen concentration in the measurement chamber to a constant value, wherein the one electrodes of the respective cells, facing the measurement chamber, are connected to the control circuit via a common wire, the method comprising the step of: diagnosing a malfunction of the common wire on the basis of a state of variations in terminal voltage at one or more of a plurality of connecting points for connecting the control circuit and the respective electrodes of the cells, wherein the connecting point is a connecting point other than a connecting point of the common wire, wherein the malfunction diagnosis of the common wire is executed in such a manner as to determine the presence or absence of occurrence of a malfunction in the common wire, based on the presence or absence of oscillations in the terminal voltage.