Air conditioning plays an important role in giving maximum comfort working and leisure environment to the occupant as well as to reduce the indoor temperature. This project describes the use of an expert system shell for air conditioning air handling unit. For air handling unit, the services and maintenances of machine are expensive due to heavily dependent on experts. Hence, the main goal of the developed system is to diagnose the problem of air handling unit. With the assistance of expert system, the diagnosis process for air handling unit can be shorten up by 566.5% and standardized compared to the conventional way. The developed system is restricted by the expert’s experiences and knowledge. A case study was conducted to verify the capability of the developed system.