Patent ID: 7045047

Claim:
A gas sensor element comprising: at least one electrochemical cell including a solid electrolyte body made of a partially stabilized zirconia; a pair of electrodes disposed on said solid electrolyte body; and a discolored portion formed in said solid electrolyte body between said electrodes, said discolored portion having a color that differs from a color of a remainder of said solid electrolyte body; wherein if a value of a limiting current produced by said electrochemical cell at 700° C. in the air is defined as I 700 , and a value of the limiting current at 900° C. in the air is defined as I 900 , a relation of 0.8×I 900 ≦I 700 ≦1.2×I 900 is satisfied, wherein the discolored portion is formed by applying a dc current to the electrochemical cell through the electrodes, wherein the voltage V of the dc current is given by the relation 1.2 V 2 ≦V≦1.5 V 2 , where V 2 is a maximum voltage within a range where a limiting current is produced in the electrochemical cell, and wherein the discolored portion occupies 80% by volume of the portion of the solid electrolyte body sandwiched between said electrodes and through which oxygen ions pass.