This paper presents a low-cost analog test system with diagnosis capabilities. The tester is able to detect faults in any linear circuit by learning a reference circuit behavior in a first step, and comparing this behavior against the output of the circuit under test in a second step. For a faulty circuit, a third step takes place to locate the fault. 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 system has been successfully applied to a case study, a biquad filter. Soft, large, and hard deviations on components, as well as faults in operational amplifiers, were considered. Experimental results have proven the feasibility and efficiency of the proposed test and diagnosis system.