The thermostat controls the flow of coolant based on engine temperature. When you first start the engine, the thermostat remains closed to quickly warm the engine to the designed engine operating temperature. After several minutes, the thermostat opens and allows a variable coolant system flow. This gives the engine the optimum operating temperature, best fuel economy, and performance for all driving conditions. Over time and use the thermostat spring weakens allowing for an inefficient non-designed flow of coolant. This can make the engine run cooler or hotter then designed. If you notice the engine temperature gauge indicating lower then normal engine temperature and your vehicle is blowing cooler air during winter months more than likely your thermostat is in need of replacement.
Also in some cases the thermostat can get stuck closed, this allows for no coolant circulation and can lead to the overheating of the engine. This can cause a blown head gasket or warped cylinder head. If your engine temperature gauge is indicating higher then normal engine temperature more than likely your thermostat is stuck closed and is in need of replacement.
