One that sticks closed produces rapid overheating.
Listings state the opening temperature, the diameter and whether a housing and gasket are included. Two thermostats that fit the same housing can open at different temperatures, and fitting the wrong one changes how the engine warms and how the computer fuels it.
On many modern engines the thermostat is integrated into a plastic housing with a sensor. Those assemblies are replaced as a unit, and the listing says so.
Replacing the thermostat is the right time to replace the coolant, because the system has to be drained anyway and old coolant degrades the new thermostat's seal.
