Abstract: An apparatus and method for adjusting the air/fuel ratio inducted into an internal combustion engine to achieve a desired air/fuel ratio. A desired fuel charge is first derived from a measurement of inducted airflow and then modulated with a triangular wave. The actual deviation of the mean value of the triangular wave from the desired air/fuel ratio is calculated. More specifically, the ratio of time the modulated signal is offset from the desired air/fuel ratio is trigonometrically related to the actual deviation. This time ratio is derived from a two-state (rich/lean) signal which is provided by comparing an exhaust gas oxygen sensor voltage output to a reference value. In response to the deviation calculation, the desired fuel charge signal is shifted to zero in on the desired air/fuel ratio.