This report on documents the results of the single-building-operator, on-line, demonstration of the Whole-Building Diagnostician, conducted at the Symphony Towers building in San Diego, California. The on-line test was designed to evaluate the Outdoor-Air Economizer (OAE) diagnostic module’s capabilities to automatically and continually diagnose operational problems with air-handling units (AHUs). As part of this demonstration, all four AHUs at Symphony Towers were monitored. The measured data that were collected on a continuous basis included: 1) outdoor-air temperature, 2) return-air temperature, 3) mixed-air temperature, 4) supply-air temperature, 5) chilled-water valve position, 6) supply-fan status, 7) outdoor-air relative humidity, and 8) return-air relative humidity.