A resistor-assembly includes a substrate, a heater, a resistor-element, and conductive-leads. The substrate is formed of a ceramic-material. The heater heats the resistor-assembly. The resistor-element is formed of an ion-conducting material that overlies the substrate. The conductive-leads are formed of a catalytic-metal that are in communication with a gas and in electrical contact with the resistor-element. The resistor-element is characterized by a resistance-value influenced by an oxygen-presence in the gas when the resistor-element is heated by the heater such that a resistor-temperature is greater than a temperature-threshold.