The faulty behavior of an open defect is determined by Byzantine effect and physical routing. Byzantine effect makes the such faulty behaviors non-deterministic and therefore, ATPG has difficulty on the fault activation and propagation which depend on both the pattern and the physical information. This paper proposes a three-stage diagnosis approach of finding combinations of open-segment defects. Path tracing extracts all candidate fault sites from error outputs of failing patterns. An ILP solver enumerates all fault combinations by considering fault candidates and simulation responses. Last, fault simulation identifies true open-segment faults by pruning false cases. Experimental results shows the resolution of the proposed approach is high and only generates <9 combination for all ISCAS 85 circuits under 1 to 5 open-segment defects.