Sensor and sensing method

A system (144) comprising a sensor (22), a reformer (146), a reformer electronic control module (150) and a sensor control module (152), wherein the sensor (22) includes an oxygen pump cell (24); an oxygen pump chamber (26); an emf cell (32); a reference chamber (34) providing a fluid connection to the reference gas (30); gas channels in fluid communication with the pump (48,52) and emf electrodes (64,66), the reference gas (30) comprising reformate produced by fuel reformer (146) fuelled by an air-fuel gas mixture having an air-fuel ratio; a heater (72); a temperature sensor (70) disposed in communication with the heater (72) and the sensor control module (152) for maintaining the sensor (22) at a desired operating temperature; a closed loop controlled operation amplifier (40) in electrical communication with the sensor (22), whereby the oxygen pump cell (24) provides sufficient oxygen ions to oxidize an incoming diffusion-limiting fuel flux to the emf cell (32) and maintain a constant emf at the emf cell (32), and wherein a current value represents an equivalent to the air-fuel ratio of the air-fuel gas mixture.