The thermostat regulates coolant temperature by starting and stopping coolant flow between the engine and radiator. Thermostats are very simple—spring pressure versus wax module (also known as the “pill”) expansion. Although the term “stuck thermostat” is common, there’s no such thing. If the wax module fails (leaks or bursts), the thermostat will not open. Temperature rating is determined by spring pressure. With a 195-degree thermostat, more heat is required to overcome the higher spring pressure, as compared to a 160 or 180 degree thermostat.