Abstract: In this paper, a fault diagnosis system is presented for analog circuit testing based on the Simulation before Test approach. The circuit under test is subjected to a frequency response analysis, from which parameters are extracted such that they give unique values for every configuration of the circuit. This uniqueness makes them ideally suitable to characterize the state of the circuit. Software implementation has been done using MATLAB. Due to manufacturing tolerances, component failures, temperature drifts and ageing, the actual values of network parameters can be different from the nominal ones and these variations may have significant effect on the circuit performances. To overcome this problem testing of the components of the circuit has been done before implementation. This work consists of analyzing the Butterworth fourth order low pass filter at different test frequencies of 89, 142 and 272 radians/second using Sheshu Waxman algorithm. to identify the fault. Further the filter parameters CDF, PDF, Normalized function are observed to analyze filter behavior using Monte Carlo Algorithm. The efficient testing and an initial fault dictionary are also developed using MATLAB software.