A split engine control system operates a multiple cylinder internal combustion engine by using only some of the plurality of cylinders under light load conditions. The total number of cylinders are split into a first cylinder group which is always activated during engine operation and a second cylinder group which is deactivated under light load conditions. The engine is provided with an exhaust passage which consists of first and second upstream exhaust passages connected to the first and second cylinder group, respectively, and a common downstream exhaust passage. An exhaust gas sensor and a first catalytic converter are disposed in the first upstream exhaust passage, and a second catalytic converter is disposed in the common downstream exhaust passage. A temperature sensor is provided in the second catalytic converter. When the temperature of the second catalytic converter is above a certain level during the partial cylinder mode, the engine is feedback controlled by a signal from the exhaust gas sensor and the exhaust gases are purified by the first catalytic converter. When the temperature is below the level during the partial cylinder mode, the air-fuel mixture is enriched with respect to the stoichiometric point so as to promote reduction of the NOx component of the exhaust gases in the first catalytic converter and oxidation of the remaining HC and CO components in the second catalytic converter.