A method for determining the degree of wear of a valve, the degree of wear of the valve being dependent on the degree of wear of an operating element that is made of an expandable material and performs a mechanical movement each time the temperature changes, the change in temperature resulting in wear, the operating element made of an expandable material being mechanically connected to a piston; the movements of the operating element (3) made of an expandable material are calculated by initially sensing the change in temperature on the operating element (3), whereupon the changes in temperature are recalculated as movements on the basis of the temperature/expansion curve applicable to the operating element (3) made of an expandable material.