the invention relates to a method for controlling an expansion valve , in particular for vehicle air - conditioning systems operated with co 2 as the refrigerant , with a valve housing which has a feed opening and a removal opening , with a valve - closing member which closes a valve seat of a passage opening arranged between the feed and removal openings , and with a resetting device which acts in the closing direction of the valve - closing member , an adjusting element , which is assigned to the valve - closing member , on the low - pressure side being activated via a predetermined threshold value as the low pressure rises or as the temperature on the low - pressure side rises and actuates at least one actuating element by means of which a closing force of the resetting device acting on the valve - closing member is reduced such that a passage opening is enlarged or a passage of a bypass valve arranged on the low - pressure side , which passage is arranged parallel to the passage opening , is opened .