For the purpose of validating fault symptoms appearing at driver outputs, a symptom validation unit is provided, within a final stage, which undertakes a validation or a preliminary validation, as applicable, on the basis of a classification of symptoms. By assigning the item of validation data "invalid" to symptoms which cannot be unambiguously identified, it is possible to suppress the further consideration of the symptoms, or to initiate further diagnoses of the final stage output concerned, as appropriate.
