Component users must customize components they obtain from providers, because providers usually develop components either for general use, or for some other specific purpose. Although the customization is accomplished by modifying the interface of a component, faults caused by customization appear when the implementation part of a component and the interfaces interact. In this paper, we select test cases by inserting faults not into the entire interface but only into specific parts of the interface, which are referred directly by the implementation. They are selected by analyzing the interaction between the interface and the implementation parts. Based on this testing approach, this paper develops a testing technique for a customized COM component. It is applied to a practical component-based system, Chamois. Through an empirical study in this paper, it is shown that the specific parts for injecting a fault brings the test cases' effectiveness, which is evaluated.