Abstract: An air-fuel control that has two stages of operation. The control includes an EGO sensor interconnected to the exhaust system of an engine and a control assembly. In a first stage of operation, the engine is allowed to operate in an open loop condition. Periodically, however, the control momentarily moves to a second stage of operation. When in the second state, the control assembly ramps up the air-fuel ratio of the air-fuel mixture supplied to the engine until the EGO sensor changes state. The amount that the air-fuel ratio changed during this process is then compared with a nominal value to compute a correction factor. The correction factor is then used during the next open loop interval to increase or decrease the signal applied to the air-fuel distribution control and achieve a more optimum air-fuel mixture.