In this paper we explore two alternative approaches to system diagnosis. The first strategy is based on testability analysis performed by SATAN tool. The second approach performed by the Sequential Diagnosis Tool (SDT) generates solutions with minimum average cost of the diagnostic tree. The application of either approach in practice depends on the goals, constraints and the available system test data. The SATAN approach is suitable for the cases where the system structure (i.e., interconnection of its functional parts) is given, while in the second approach the probabilities of faulty states and the test matrix implicitly describe the functionality of the diagnosed system.