A gas sensor is provided for detecting one or more gases in a gas sample. The gas sensor includes a substrate, a solid electrolyte layer including lanthanum oxide for sensing carbon dioxide, a heating element thermally coupled to the solid electrolyte layer, and a controller coupled to the heating element and the solid electrolyte layer. The controller heats the heating element so that the solid electrolyte layer reaches an operating. Methods of sensing carbon dioxide and humidity are also disclosed.