Why Zirconia Oxygen analyzer needs Furnace to warm up before they can show oxygen ppm level

The Zirconia sensor is a high temperature electrochemical type.  The Zirconium oxide, treated with Yitium oxide and plated with platinum on opposite ends acts as the sensor.  At elevated temperature, pores are formed in the zirconia lattice allowing oxygen movement from higher concentration to lower concentration, based on the partial pressure of oxygen.  To create this partial pressure differential, one electrode is usually exposed to air (20.9% oxygen) while the other electrode is exposed to the sample gas.  The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference gas and sample gas.