Abstract: In order to repair faulty line interface boards in SPC switching system AXE-10, we developed a tester to locate the faults on boards. The efficiency of the tester can be improved by the optimization of its original test flowchart. However, an overall optimization takes a lot of time and needs great efforts. In this paper, a local-optimization method oriented to frequently occurring faults is presented. A heuristic fault diagnosis algorithm with fewer probing operations is also discussed.