Optimal performance of the valve can be verified by conducting a simple Cold Water Failure Test. With the tap running, turn the cold water supply off at the isolating valve. The flow at the tap should immediately stop and then, as the element relaxes, dribble a tiny amount of warm water. Closing the cold water supply suddenly bathes the thermostat element in very hot water, which causes it to expand and moves the slide valve to shut off the hot water supply.
A valve will require maintenance where the flow does not reduce to zero followed by an intermittent dribble.