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Composite NO_x sensors were fabricated by combining partially and fully stabilized yttria-doped zirconia with alumina forming a composite electrolyte, Y₂O₃-ZrO₂-Al₂O₃, and strontium-doped lanthanum manganese oxide mixed with gold to form the composite sensing electrode, La_0.8 Sr_0.2MnO₃-Au. A surface chemistry analysis of the composite sensor was conducted to interpret defects and the structural phases present at the Y₂O₃-ZrO₂-Al₂O₃ electrolyte, as well as the charge conduction mechanism at the LaSrMnO₃-Au electrode surface. Based on the surface chemistry analysis, ionic and electronic transport properties, and microstructural features of sensor components, the working principle was described for NO_x sensing at the composite sensor. The role of the composite materials on the NO_x sensing response, cross-sensitivity to O₂, H₂O, CO, CO₂, and CH₄, and the response/recovery rates relative to sensor accuracy were characterized by operating the composite <inline-formula><math display="inline"><semantics><mrow><msub><mrow><mi>NO</mi></mrow><mi mathvariant="normal">x</mi></msub></mrow></semantics></math></inline-formula> sensors via the impedimetric method. The composite sensors were operated at temperatures ranging from 575 to 675 °C in dry and humidified gas environments with NO and NO₂ concentrations varying from 0 to 100 ppm, where the balance gas was N₂. It was found that the microstructure of the composite NO_x sensor electrolyte and sensing electrode had a significant effect on interfacial reactions at the triple phase boundary, as well as the density of active sites for oxygen reactions. Overall, the composite NO_x sensor microstructure enabled a high NO_x sensing response, along with low cross-sensitivity to O₂, CO, CO₂, and CH₄, and promoted NO detection down to 2 ppm.