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 first 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. A feedback control circuit is responsive selectively to a signal from the first gas sensor during the partial cylinder operation for correcting the ratio of mixture supplied to the second group cylinders and to a signal from the second gas sensor during the full cylinder operation for correcting the ratio of mixture supplied to all the cylinders.