In electrochemical CO$_2$ gas sensor, the chemical potential of electrolyte changes on adsorption of CO$_2$ molecules as the process involves catalytic electron transfer. In addition, it is the rate-determining step that decidessensor’s response. In this study, in-situ bulk AC, DC and surface electronic conductivities of CaCO$_3$ + Li$_2$CO$_3$ binary solid electrolyte were investigated at different temperatures and CO$_2$ gas partial pressures using complex impedance spectroscopy, Wagner’s DC polarization technique and four-probe method, respectively. For the four-probe conductivity measurements with crucial requirement of high temperatures and test gas variations, a customized sample holder was designed and fabricated having gold-plated equidistant, spring-loaded electrodes and localized heating system (maximum 593 K). The AC bulk conductivity was found to decrease with rise in CO$_2$ gas concentration (from 0.1 to 100%) by about two orders and one order of magnitudes at lower and higher temperatures, respectively. Similarly, surface conductivity variation with temperature also showed Arrhenius behaviour for both the concentrations of CO$_2$ viz. 0.04 and 10%, giving lower value of activation enthalpy for lower CO$_2$ concentration. The surface conductivity change in the presence of different concentrations of CO$_2$ gas is justified by comparing with AC bulk conductivity measurements at different CO$_2$ partial pressures and DC conductivity along with sensing response. The mechanism is explained using activated charge transfer data. The range of $E_a$ values on adsorption of CO$_2$ gas was found to be in the electronic excitation window, suggesting involvement of a new parameter to be investigated for non-Nernstian response of EC sensors.