Troubleshooting of an air-handling unit (AHU) is a complicated process and requires high level of expertise. Any attempt of developing an expert system dealing with troubleshooting in AHU has to overcome various difficulties. This paper describes the implementation of AHUX, a knowledge-based expert system (KBS) for diagnosis the trouble of a typical AHU and for remedy. AHUX is to teach troubleshooting of AHU for the students and the non-specialized technical staffs. The system structure, its components and their function are described. The system has only 10 rules for loading a fact file and reaching a result for troubleshooting. The fact file has four types of fact acting as a rule. The process of diagnosis is leaded by the symptom. Each of symptoms indicates one or more of the assumption are false. The system has been tested and gave promising results. The AHUX and the Fault Analysis Table were compared and emphasized the motivation effect on training. The result of the evaluation was accurate and promising.