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We show that SiC based metal-insulator-semiconductor (MIS) sensors above 700 K respond to reducing/oxidizing gases by two independent mechanisms: a chemically induced shift in the metal-insulator barrier height and the passivation/creation of charged states at the SiO2-SiC interface. The relative magnitude of the two phenomena is determined by the position of the Fermi energy at the insulator-semiconductor interface, i.e. the capacitor bias. The interface state contribution is much slower than the work function shift. For short response times and high sensor-to-sensor reliability, we find that the optimal operating bias of a gas sensor is near midgap. The combination of photoemission measurements and capacitance measurements can discriminate between the two sensor mechanisms.