O2 Sensor, apparatus for and method of controlling air-fuel ratio of an internal combustion engine, and recording medium storing air-fuel ratio control program

A system controls the air-fuel ratio in an exhaust gas supplied from an internal combustion engine to a catalytic converter in order to keep an output voltage Vout of an Osensor disposed downstream of the catalytic converter at a predetermined target value Vop. In the system, the temperature of an active element of the Osensor is controlled at a predetermined temperature such that the output voltage of the Osensor at an inflection point of output characteristics thereof is substantially the same as the target value Vop.