In order to ensure high quality of DSM circuits, testing for the open defect in the circuits is necessary. However, the modeling and techniques for test generation for open faults have not been established yet. In this paper, we propose a method for generating tests and diagnostic tests based on a new open fault model. Firstly, we show a new open fault model with considering adjacent lines . Under the open fault model, we reveal more about the conditions to excite the open fault. Next we propose a method for generating tests for open faults by using a stuck-at fault test with don't cares. We also propose a method for generating a diagnostic test that can distinguish the pair of open faults. Finally, experimental results show that 1) the proposed method is able to achieve 100% fault coverages for almost all benchmark circuits and 2) the proposed method is able to reduce the number of indistinguished open fault pairs.