Under the modern manufacturing technologies, the open defect is one of the significant issues to maintain the reliability of DSM circuits. However, the modeling and techniques for test and diagnosis for open faults have not been established yet. In this paper, we give an important clue for modeling an open fault with considering the affects of adjacent lines. Firstly, we use computer simulations to analyze the defective behaviors of a line with the open defect. From the simulation results, we propose a new open fault model that is excited depending on the logic values at the adjacent lines assigned by a test. Next, we propose a diagnosis method that uses the pass/fail information to deduce the candidate open fault. Finally, experimental results show that the proposed method is able to diagnose the open faults with good resolution. It takes about 6 minutes to diagnose the open fault on the large circuit (2M gates).