Abstract: Devices, methods, and systems for determining a cause of a fault in a heating, ventilation, and air conditioning (HVAC) system are described herein. One device includes a memory, and a processor configured to execute executable instructions stored in the memory to receive operational data associated with an HVAC system, store the received operational data in a database, determine, upon a fault occurring in the HVAC system, a cause of the fault by applying the operational data in the database to a table that maps causes of possible faults that may occur in the HVAC system and symptoms corresponding to each respective possible fault, and provide the cause of the fault occurring in the HVAC system to a user.