Outdoor air economizers (OAE) control the amount of air drawn into a building in order to reduce energy usage and improve indoor air quality. Air-side economizers are required by energy standards such as ASHRAE Standard 90.1 in many commercial applications because of their energy savings’ potential. However, many economizers do not operate as designed because of damper, sensor, and other faults which can actually lead to greater energy use. Fault detection and diagnostics (FDD) tools have been developed to identify faults when they occur, however, no general methodology for assessing these tools has yet to be proposed. This paper presents and applies a general OAE FDD assessment method. FDD tools described by Seem and House (2009), Schein et al. (2003), and Brambley et al. (1998) are analyzed with respect to various OAE faults. Additionally, these FDD methods are assessed with the method using simulated and experimental data with both single and multiple simultaneous faults.