A method is disclosed for controlling operation of a engine coupled to an exhaust treatment catalyst. Under predetermined conditions, the method operates an engine with a first group of cylinders combusting a lean air-fuel mixture and a second group of cylinders pumping air only (i.e. without fuel injection.) In addition, the engine control method also provides the following features in combination with the above-described split air / lean mode: auto speed control, sensor diagnostics, air-fuel ratio control, adaptive learning, fuel vapor purging, catalyst temperature estimation, and default operation. In addition, the engine control method also disables the split air / lean operating mode under preselected operating conditions.