An impedance method for the periodic ‘in situ’ diagnostics of the solid electrolyte/liquid-metal electrode interface during the lifespan of yttria-stabilized zirconia (YSZ) based sensors measuring oxygen partial pressure in melts was developed. Polarization effects on the YSZ, resulting from the corrosive measuring environment (molten alkaline metals), may be interpreted as a blocking reaction layer on the electrolyte/liquid-metal electrode interface. The proposed impedance method allows information to be obtained about the level of polarization of the YSZ/liquid-metal electrode interface that is characterized by a second semi-arc of a hodograph of impedance. The second semi-arc represents the parameters of polarization resistance (Rf) and capacitance of the double electrical layer on the interface YSZ/liquid metal (CD). Analysis of the impedance method on the single crystal zirconia sensor measuring dissolved oxygen in molten lead at temperatures of 380–480 °C revealed that this sensor, with a Bi–Bi2O3 reference electrode, showed a negligible level of polarization effects on the electrode/electrolyte interface at temperatures as low as 380 °C. The results of the present work may be applicable for the diagnostics of oxygen sensors with more complicated applications, such as in the measurement of oxygen activity in lead–bismuth, sodium or lithium heat carriers in liquid-metal nuclear facilities.