In this paper, we propose an effective diagnostic method in the presence of an unknown fault model, based on only pass/fail information on the applied tests. The proposed method deduces faulty conditions that are able to explain the behavior of the defect in the circuit and locates faulty sites, based on the number of detections for the single stuck-at fault at each line, by performing single stuck-at fault simulation with both passing and failing tests. As a result, we can derive a fault model from the faulty condition. To improve the ability of fault diagnosis, our method uses the logic values of lines and the condition whether the stuck-at faults at the lines are detected or not by passing and failing tests. Experimental results show that our method can accurately identify the fault models for 93% of faulty circuits and that the faulty sites are located within several candidates except for circuits with multiple stuck-at faults.