an engine air / fuel control system and method is provided having feedback control responsive to an exhaust gas oxygen sensor . a fuel command responsive to the ego sensor is modulated and the modulation disabled in response to detection of an air / fuel transient period . in addition , feedback control gain is increased and the modulation signal held at a value opposite the state of the ego sensor occurring upon initiation of the air / fuel transient period . an indication is provided that the air / fuel transient is terminated when the ego sensor switching frequency resumes a normal condition . modulation is thereafter resumed and the feedback gain restored .