COOLING SYSTEM
The cooling system is responsible for regulating the engine temperature within a workable limit, majority of cars engines are water cooled that is they use coolant. The cooling system consists of the Radiator, Water Pump, Hoses, Thermostat, and Fans. Air is forced by the fan to cool the coolant in the radiator; this coolant is pumped by the water pump through the hoses down to the pores in the engine block where heat is transferred by convention through the water back to the radiator to be cooled again.
The fan is controlled by the thermostat which put it on when the temperature of the engine rises to a certain level and put it off when the temperature falls. When there is a problem in the cooling system of the vehicle it could lead to overheating. Overheating can cause serious damage to the engine which including burning of head gasket.

Overheating can be caused by;
Cooling fan not working properly.
Faulty thermostat.
Faulty water pump.
Blocked hoses or engine water pores.
Blocked or leaky radiator.
There are hardly air cooled cars these days, but motorcycles engines are usually air cooled they don’t need any component or system for the cooling. The transfer of heat is by convention to the atmospheric air.
Thermostat
The thermostat is a component that helps control the flow of coolant through the engine to aid cooling. When the temperature of the engine is high, it requires coolant to come in and maintain a healthy temperature for the engine. The thermostat helps to allow for the following in of coolant, by opening itself, when the engine temperature reaches a certain level.
I have experience series of repairs when a thermostat was assumed to be faulty, but after replacement, it was discovered that it was a fuse that was bad. A vehicle can also operate without a thermostat, but this means that the supply of coolant into the engine would not be controlled by temperature and would commence once the engine is turned on.
One major way of knowing if your thermostat is bad is by removing the suspected thermostat and check if the radiator or condenser supplies coolant directly, if it does then the thermostat is bad, this is when all other components have been discovered to be in proper working conditions.
Better still, using good Diagnostics tools and options, can save you the stress of making wrong Diagnostics and reduce guesses. A faulty thermostat if not replace can lead to Engine Overheating, which can cause several damages to the engine as well as Engine Knocking.