Recently, a design-for-test (DFT) methodology for active analog filters was proposed with primary goal in increased controllability and observability. We operationalize and extend the DFT methodology by using CLP(R) to model and diagnose analog circuits. CLP(R) is a logic programming language with the capability to solve systems of linear equations and inequalities. It is well suited to model parameter tolerances and to diagnose soft faults, i.e., deviations from nominal values. The diagnostic algorithm uses different DFT test modes and voltage measurements at different frequences to compute a set of suspected components. Ranking of suspected components is based on a measure of (normalized) standard deviations from predicted mean values of component parameters. Presented case studies on real circuit show encouraging results in isolation of soft faults for a given low pass biquad filter.