Although open defects are hard to diagnose because they are unstable, we developed a technique to diagnose completely open defects. We applied a new "segment model" that takes the coupling effects on a defective node that are caused by neighboring nodes into consideration. This technique is used to focuse not only on the behavior of the defective node, but also on the behavior of other nodes affecting its behavior. We explain the theoretical treatment of our model and present experimental results obtained from an actual chip.