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The Lennox 12J39 (103366-11) is a genuine OEM expansion valve engineered for Lennox NXT series outdoor units at 2.5-ton capacity. It controls the flow of refrigerant into the evaporator to ensure precise temperature and pressure regulation across the refrigerant circuit. Signs of a failing expansion valve on your Lennox NXT unit include loss of cooling capacity, high suction pressure, or erratic system cycling.