A method is provided for operating a cooling system of an internal combustion engine, which cooling system has a controllable rotary slide valve with at least one switched inlet or outlet. The movement of the rotary slide valve into a plurality of switching positions, which each correspond with a cooling system state, is monitored. In accordance with an improper functional state of the rotary slide valve and a current switching position of the rotary slide valve, an operating state of the internal combustion engine is changed to an emergency operation state. A protection system in the cooling system carries out the method and includes a thermal management system, which receives and processes coolant temperatures, and a control unit of a controllable rotary slide valve having a position detector, which can detect a current switching position of the switchable rotary slide valve, wherein the thermal management system is connected to the control unit of the rotary slide valve.