It is well known that the tests and diagnostics influence greatly the communication software reliability. The testability and the easiness of the diagnostic process of communication software are becoming a major concern of the design community. The fault detection and the fault localization problems are strongly related issues. The easiness of diagnostics can be seen as a criteria of testability, it is in fact characterized by specific requirements at the design level. In this paper, we will make a clear link between these two issues and we will study the influence of some testability factors on the diagnostic activity. We will present our results and hints on "diagnosablility" in the context of finite state machine model.