P-type semiconductor-based gas sensors have been fabricated for the selective detection of 50–200 ppb nitric oxide (NO). To fabricate the sensors, cobalt oxide (Co3O4) powders containing a noble metal (Pd, Ag, Pt, or Au) at 10 wt% were prepared by a colloidal mixing method and deposited on interdigitated Pt electrodes. The sensing responses to 50 and 200 parts per billion (ppb) NO and 25 and 100 parts per million (ppm) H2 in air were investigated. The sensors with 10 wt% Pd, Ag, Pt, or Au loaded on the Co3O4 powder demonstrated a greater response to NO than Co3O4 with no noble metal content. The addition of Ag was found to suppress the H2 response, a phenomenon that is attributed to the surface modification effects of Ag on the Co3O4 surface. Furthermore, the sensor with 5 wt% Pd and 5 wt% Ag loaded on the Co3O4 showed a high response in the presence of 50 ppb NO, as well as good selectivity against H2.