While a number of different approaches have been proposed to automatically troubleshoot electronic systems given schematic information, few are sufficiently powerful to tackle the complexity of analog circuits at the resistor/transistor level. This paper describes work which applies quantitative modelbased reasoning techniques to this problem. The circuit schematic is converted into a constraint diagram to which the combination of constraint propagation and dependency tracking are applied to search for inconsistencies and identify the implicated components. Instead of resorting to the propagation of symbols, the technique of aggregate models is used to enhance deductive power but with manageable computation. These ideas have been implemented in the program FIX which diagnoses a given circuit by recognizing inconsistencies among measurements, identifies the set of fault candidates and their posterior probabilities, and suggests the best next measurement. Modeling and inference issues are discussed, and diagnosis of various faults in an example circuit by FIX is described.