There are many kinds of faults in the actual analog circuits. And the limited number of test nodes results in a possible overlap between the different fault classes. For the fault overlap, the usage of conventional diagnostic methods based on Mahalanobis distance makes the high misdiagnosis rate. This paper describes the serious consequences of misdiagnosis for late circuit improvement, and it is significant to enhance the accuracy of fault diagnosis by reducing the faults resolution. It is proposed that not only the optimal value but also the degree of optimization and inferior between the optimal and suboptimal. The fault diagnosis based on Mahalanobis distance is combined to explain it concretely. Finally, the analysis of the specific circuit is done to verify the validity of the method in this article.