An internal combustion engine operable in variable displacement modes is provided with separate exhaust systems for individual cylinder groups and a set of exhaust gas sensor and catalytic converter for each exhaust system. An electronic fuel injection control unit is arranged to respond to one of the signals from the gas sensor which is indicative of mixture richer than the other to effect correction of the duration of fuel injection pulse. Each catalytic converter is provided with a temperature sensor to detect when the temperature therein falls below the normal catalytic reaction temperture. A variable displacement control unit responds to the output from the temperature sensors by alternately deactivating the cylinder groups during low power operation.