Logic diagnosis is the process of isolating the source of observed errors in a defective circuit, so that a physical failure analysis can be performed to determine the root cause of such errors. In this paper, we propose a new “Effect-Cause” based intra-cell diagnosis approach to improve the defect localization accuracy. The proposed approach is based on the Critical Path Tracing (CPT) here applied at transistor level. It leads to a precise localization of the root cause of observed errors. Experimental results show the efficiency of our approach.