Abstract: An application shell provides a domain independent diagnostic problem solver comprised of a knowledge base. The knowledge base includes a plurality of failure mode schemata arranged in a causal hierarchy from observable failure modes to failure modes having a repair procedure; a plurality of rule schemata related to certain of said failure mode schemata; a plurality of datum schemata for gathering data about the system under investigation, the information being used to determine the applicability of said rule schemata; and a plurality of repair procedure schemata each being related to one of the failure mode schemata having a repair procedure.