An aircraft environmental control system (ECS) includes a first valve that is adjustable between a plurality of first positions to open or shut-off a bleed air flow that flows from an inlet of the first valve to an outlet of the first valve. A second valve includes an inlet in fluid communication with the first valve outlet, and is adjustable between a plurality of second positions to throttle the bleed air that flows from the inlet of the second valve to an outlet of the second valve. A valve position sensor outputs a position signal indicating a measured position among the plurality of second positions. A controller is in signal communication with the valve position sensor. The controller is configured to determine a primary effective area of the second valve based on the measured position, and to diagnose operation of the first valve based on the primary effective area.