Many operational problems that are identified during retro-commissioning (RCx) can be detected automatically and continuously, allowing the building to operate near optimally, leading to a lower RCx cost and increased persistence. A set of algorithms and software code to detect and diagnose faulty operations with economizer operations have been developed. The algorithms use rules derived from engineering principles of proper and improper for rooftop and air-handling unit operations. The algorithms include: ? Detect AHU sensor faults (outdoor-air, mixed-air, and return-air temperature sensors). ? Detect if the AHU is not economizing when it should. ? Detect if the AHU is economizing when it should not. ? Detect if the AHU is using excess outdoor air. ? Detect if the AHU is not providing sufficient ventilation air.