A Nissan thermostat is a device that regulates the operating temperature of your engine. This cooling system component is located at the exit portal on the engine block, and has the upper radiator hose attached to its housing cover. The exact temperature that the thermostat will open at is determined by the automaker. It consists of a housing that holds wax pellet material, a piston, and a spring. When it's cold, the hard wax forces the piston outward and that pushes the spring and flange down to block coolant from entering the radiator. As the coolant temperature increases, the wax will expand and the spring will relax. This opens the flange and allows coolant into the radiator. This is how the thermostat regulates the engine temperature when in place on your Nissan. The different temperature settings of the Nissan thermostat are typically available in 5 degree Fahrenheit increments. Unless the owner of the vehicle has an extremely good reason for changing this preset operating temperature of their engine, the one set by the factory should also be the replacement part when this cooling component fails. A thermostat can fail two ways. Either the valve will not open or the valve will remain open no matter what the engine temperature is. In either instance, the Nissan engine will not be able to perform at its optimum level of efficiency when this occurs. When either of these conditions occurs, the thermostat should be replaced. If the Nissan is driven when the valve is jammed open, the temperature will never reach its proper operating level and the mileage and performance will be adversely affected. If the valve is jammed shut, the engine will overheat in less than 5 minutes of it running and will have to be turned off or severe damage to the interior components of the engine will occur.