Epitaxial 4H-SiC graphene films for use in ambient gas sensing are fabricated and tested. The sensitivity response to nitrogen dioxide is optimized by varying both operation temperatures and humidity. A relative resistance change response of -45% is obtained upon application of elevated temperatures and a gas mixture containing NO₂ at a concentration of 10 parts per billion (10 ppb). The sensitivity response increased linearly with NO₂ concentration, reaching -60% at a concentration of 250 ppb, followed by saturation at 1 part per million (ppm) level.