A sensor element including an insulative base, a heating resistor, an oxygen pump cell, an oxygen detection cell, and a diffusion-chamber-forming member having a rate-controlling introduction portion and defining a diffusion chamber in cooperation with electrodes, wherein these components have been subjected to simultaneous firing to form a unitary sensor element. The method comprises the steps of receiving a signal, demodulating the signal, generating an ideal signal from the demodulated signal and estimating the performance characteristic from a difference between the ideal signal and the received signal. 5\xb0 smaller than the plate angle.