Replacement hardware to suit Southern Star™ patio doors comes down to a single part here: a 25kg adjustable patio door carriage and roller. That it is the only listing is itself the answer to what fails, because on a sliding patio door the carriage takes the whole weight of the leaf, and the wheel wears flat or the bearing seizes long before the track, the lock or the handle need attention. The symptom is a door that needs both hands and scrapes at one end. The adjustment screw is what you use to lift the leaf back level once the new carriage is in. Before ordering, take the old carriage out and check the body length, the roller diameter and how it seats into the bottom rail, and confirm the leaf is within the 25kg rating.