1. A method to diagnose a failing system element causing a symptom in a system-under-test, said method comprising the steps of: a. defining a diagnostic symptom tree comprising a plurality of symptom roots and a dependent plurality of lower nodes, the plurality of lower nodes comprising a plurality of function nodes depending from the symptom roots, a plurality of sub-function nodes depending from the function nodes, a plurality of element nodes depending from the function or sub-function nodes, and a plurality of penultimate failure mode leaves depending from the element nodes; b. associating test information with each of the function and sub-function nodes and the failure mode leaves; c. selecting one of the plurality of symptom roots responsive to identifying a symptom; d. performing the test information associated with the selected symptom root, thereby generating a symptom root test result; e. responsive to a positive symptom root test result: e1. performing the associated test information of each of the symptom tree depending nodes and failure mode leaves responsive to each positive test result; and e2. identifying a function or sub-function or element node associated to a lowest symptom tree node or failure mode leaf for which a test is positive; and f. if no test result is positive, advising that no failing function or sub-function or element is found.