The modern power electronics is indistinctive to questions on a reliability. The electrolytic capacitors despites their excessive properties in terms of affordability, power density to volume and weight, remains an element that can cause device fail. The paper is dedicated to a solution to diagnostics, monitoring and prediction of remain lifetime of electrolytic capacitors. The main idea is that ESR (Equivalent Series Resistance) of capacitor grows while capacitor is getting to the end of the lifespan. ESR in combination with the capacitance is forming a Left Half-Plane zero and thus change in listed parameters is affecting overall transfer function. This paper uses buck converter as an example. The data for capacitors models is obtained from real accelerated aging experimental results. Modelling at different lifespan periods shows change in response to disturbance. The paper proposes to use band-pass filter to get signal that is easy to process further. The paper also has a little discussion on potential uses of the signal and their advantages and disadvantages.