The present invention provides a fault diagnosis device for optimizing maintenance measures in a system, in particular a flying object. The diagnosis device has a storage unit for storing diagnosis rules, which each have a fault condition indicating the presence of a fault, and a fault conclusion formed by logical combinations of potential fault causes of the respective fault. Furthermore, a logic unit is provided, which logically combines those stored fault conclusions whose associated fault conditions are met and, on the basis of the combined fault conclusions, determines a logical diagnosis result having at least one fault cause hypothesis. A calculation unit subsequently calculates an occurrence probability for each fault cause hypothesis of the logical diagnosis result determined.