Abstract: A managed fuel air mode control system is provided which operates in an open loop lean mode of operation whenever open loop enable criteria is met. During the open loop lean mode of operation, a rich A/F purge mode of operation is dictated when a NO.sub.x trap is determined to be full. After purging the trap, engine operation returns to stoichiometric and either enters a closed loop learning operation or returns to lean operation depending upon the time that has passed since the last learning operation. During the learning operation, an adaptive control algorithm learns or updates a long term correction factor that is used during the open loop lean A/F mode of operation. Also during open loop operation, the amount of SO.sub.x accumulated in the trap is calculated and the open loop mode is terminated when the amount of SO.sub.x exceed a threshold.