The monovalve defaults to full heat. It is possible that the solenoid is bad and not pulling the valve shut to restrict the hot water from flowing through the heater core. If the solenoid is activating the rubber diaphram inside the valve could be torn.// It is not too awfully difficult if I remember corectly to remove the top half of the valve-leaving the coolant hoses connected- and inspect the diaphram and bench test the soleniod