A diagnostic tool for an energy conversion appliance, which may be a heating, ventilating or air conditioning appliance, includes a sound receiving transducer (1) coupled to output elements (5) to identify, discriminate and log the acoustic signatures of a sequence of events within an operating sequence of an appliance. Individual event occurrences are identified by comparison of a received acoustic signature with stored data to determine whether an event occurrence has occurred in a proper manner and at a correct position in the sequence of a plurality of event occurrences.