This work presents a new diagnosis method for use in an adaptive analog tester. The tester is able to detect faults in any linear circuit by learning a reference behaviour in a first step, and comparing this behaviour against the output of the circuit under test in a second step. Considering the same basic structure, the diagnosis method consists on injecting probable faults in a mathematical model of the circuit and later comparing its output with the output of the real faulty circuit. This method has been successfully applied to a case study, a biquad filter. Component soft, large, and hard deviations, and faults in operational amplifiers were considered. The results obtained from practical experiments with this analog circuit are discussed in the paper.