A control system for a multi-cylinder internal combustion engine comprises a split engine control unit for operating the engine on partial cylinders when reduced engine power can adequately operate the vehicle, and first and second exhaust conduits for directing exhaust gases from first and second group cylinders respectively to a common junction to which a common exhaust conduit is connected for directing the gases to the atmosphere. A first set of an exhaust gas sensor and catalytic converter is disposed in the second conduit to be exposed to the stream of gases exhausted from the second group cylinders which are activated at all times and a second set of an exhaust gas sensor and catalytic converter is disposed in the common conduit to be exposed to the stream of gases exhausted from the first group cylinders directed through the first conduit and to the gases passed through the upstream side catalytic converter in the second conduit. Feedback mixture control operation is effected on the basis of the signal from the first exhaust gas sensor during the partial cylinder mode and a full cylinder mode in which the second gas sensor is still below its normal operating temperature. The signal from the second gas sensor is utilized for the feedback operation in the full cylinder mode in which the temperature of the latter sensor is above its normal operating point.