In this paper, we consider the problem of detecting faults within a component of a system of communicating FSMs. These faults are detected by a given set of external test cases, which satisfy appropriate test purposes. Given a component FSM and an external test case, we propose a procedure for deriving the set of all output faults of the component machine, which can be detected with this test case. Applications range from test suite minimization to derivation of diagnostic tests for a component FSM. In this paper, we use the above technique for augmenting a given test suite in order to detect all output faults within a component under test. The same approach can be used with respect to any other finite set of faults within the implementation of a component FSM.