Abstract: A method is disclosed for controlling operation of an engine coupled to an exhaust treatment catalyst. Under predetermined conditions, such as after an engine cold start, the method operates an engine with a first group of cylinders having a first ignition timing, and a second group of cylinders having a second ignition timing more retarded than the first group. In addition, the engine control method also provides the following features in combination with the above-described split air/lean mode: idle speed control, sensor diagnostics, air/fuel ratio control, adaptive learning, fuel vapor purging, catalyst temperature estimation, default operation, and exhaust gas and emission control device temperature control. In addition, the engine control method also can change to combusting all cylinders at substantially the same ignition timing under preselected operating conditions such as fuel vapor purging, manifold vacuum control, and purging of stored oxidants in an emission control device.