Patent ID: 7636624

Claim:
A gas sensor diagnostic method of diagnosing an improper condition of a gas sensor for producing a gas sensor output representing a concentration of a specified gas component in an exhaust gas mixture of an internal combustion engine, comprising: a sensor output obtaining step of obtaining sensor output values of the gas sensor output at regular time intervals of a predetermined obtaining period; an interruption detecting step of detecting a fuel supply interruption to the engine; an extremum determining step of determining local extreme values each of which is an extremum in a series of sensor output values obtained consecutively by the sensor output obtaining step during a fuel supply interruption period during which the fuel supply interruption is detected by the interruption detecting step; an extremum comparing step of comparing each of the local extreme values with a first predetermined threshold level; and a diagnosing step of determining whether the gas sensor is in the improper condition or not, in accordance with results of the comparison of the local extreme values with the first threshold level when a number of the local extreme values obtained from the gas sensor output becomes equal to a predetermined first number.