Air-fuel ratio detecting device and method

A method for detecting an air-fuel ratio of a combustion engine using:

• an air-fuel ratio sensor composed of a solid electrolyte which is disposed in the exhaust system of an internal combustion engine and passes an electric current when a voltage is applied thereto;
• an air-fuel ratio sensor circuit which applies a voltage to the air-fuel sensor, detects the current and generates an output that varies in proportion to the magnitude of the current;
• and a storage means for storing the data of a conversion map which is used for calculating the air-fuel ratio of the internal combustion engine in response to the output of the air-fuel ratio sensor circuit, said air-fuel ratio detecting method comprising the steps of:
• determining whether the air-fuel ratio sensor is in an inactive state;
• a circuit error detecting step in which an error in the output caused by the air-fuel ratio sensor circuit is detected as a difference between a reference output value and an output value of the air-fuel ratio sensor circuit when the air-fuel ratio sensor is in the inactive state, and
• a map calibration step in which the data of said conversion map is calibrated so that an air-fuel ratio indicated by said conversion map is calculated based on the output value of the air-fuel ratio circuit and the error detected by said circuit error detection means.