We propose a new Simulation-After-Test (SAT) method-ology for accurate diagnosis of circuit parameters in large analog circuits. Our methodology is based on constructing a non-linear regression model using prior circuit simula-tion, which relates a set of measurements to the circuit's internal parameters. First, we give algorithms to select measurements that give all the diagnostic information about the Circuit-Under-Test (CUT). From these selected measurements, we solve for the internal parameters of the circuit using iterative numerical techniques. The method-ology has been applied to several mixed-signal test bench-mark circuits and has applications in process debugging for mixed-signal integrated circuits (ICs) as well trouble-shooting and repair of board level systems.