Traditionally, in fault diagnosis systems, the user is instructed to investigate symptoms exhaustively until a single fault is identified. A more advanced known system recognises that it may be cost effective to address a fault that has been determined as being likely but not certain to exist; in preference to further examination of the symptoms. However this technique has been found not to work well when a symptom is known to be only sometimes associated with a fault. The invention addresses this problem by 1) deriving a first value, for each fault, of probable benefit of acting on that fault and for identifying the fault for which that value is greatest, 2) deriving a second value, for each symptom, of probable benefit of an investigation into that symptom and for identifying the symptom for which that second value is greatest, and 3) comparing the greatest first value with the greatest second value thereby determining when to switch from investigating symptoms to acting upon a fault. By employing the invention the aforementioned problem can be overcome because the system calculates the extent to which each symptom examination would improve the situation, assuming no further symptom investigations are used.
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