A method and system comprising a diagnostic symptom tree system for diagnosing a failing system element causing a symptom in a system-under-test. A diagnostic symptom tree comprises symptom roots and dependent lower function nodes and sub-function nodes. Element nodes depend from the function or sub-function nodes, and a plurality of penultimate failure mode leaves depend from the element nodes. The function and sub-function nodes and the failure mode leaves comprise test information. Responsive to positive test results, the diagnostic symptom tree is configured to identify 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, or advise that no failing function or sub-function or element is found. In one aspect of the invention, nodes may include parameter values allowing successive selections of the nodes of the symptom tree for test iterations according to the parameter values.