Fault identification of analog and mixed signal circuits is very difficult topics for previous few decades. Localization of hard and soft faults in analog circuit is often a troublesome task without a transparent cut methodology. For manufacturing process of Very Large Scale Integration (VLSI) Application Specific Integrated Circuits (ASICs) both fault diagnosis and localization are mandatory. The importance of such analog test has become important due to enhancement of networking and communication sector. This paper gives a brief review on the faults present in analog circuits and different diagnosis methodologies of these faults. Comparison of detection is also demonstrated among some techniques for some of the ITC97 Benchmark circuits.