An emissions control system is provided that comprises a fuel tank, a hydrocarbon filter element, and a three-way valve coupled between the hydrocarbon filter element and the atmosphere. The three-way valve is comprised of three ports, a purge port, a reference port, and a sealed port, and enables identification of degradation of the emissions control system. The arrangement of the ports on the valve enables a simplified testing method that reduces background errors caused by transitioning between various operating positions.