Engine air/fuel ratio is controlled in response to a comparison of an exhaust gas oxygen sensor output with a reference value. A correction voltage is generated which is related to a change in the midpoint between saturated output states of the sensor during a test period in which engine air/fuel operation is first forced rich and then forced lean of a preselected air/fuel ratio. The sensor output amplitude is shifted with the correction voltage to reduce variations between the reference value and the midpoint.