Abstract: A solid electrolyte gas analyzer is equipped with a diagnostic circuit that intermittently measures an impedance of the solid electrolyte measurement cell as an indication that the cell is in need of calibration or replacement. The diagnostic operation is performed on the cell in situ, i.e., while the solid electrolyte cell is exposed to the gas to be measured and heated to a controlled elevated temperature. Preferably the cell impedance is measured at a predetermined frequency by injecting a modulated current through the cell and measuring the resulting voltage drop.