A novel mode of operation of a double-chamber zirconia oxygen sensor is presented. A pseudo-reference gas was generated in one chamber of the sensor to pin the potentials of the electrodes acting to pump oxygen to or from the other chamber. The circuitry was simple and readily implemented. The sensor was tested in the flue of a gas-burning system and shown to provide an output which enabled the normalized air-to-fuel ratio, lambda, to be measured over the range 0.7 < lambda < 1.7. The characteristics are discussed from a theoretical standpoint.