A facile, lithography-free fabrication strategy based on the face-to-face (F2F) assembly of carbon nanotube (CNT) forest blocks conformally coated with cerium dioxide (CeO₂) is demonstrated for a gas sensor operable at room temperature. A thermally stable TiN base electrode provides an electrically conductive and heat-resistant foundation during the atmospheric CVD growth of vertically aligned CNT forests on an Fe catalyst layer, while the CeO₂ coating introduces an n-type semiconductor interface that forms a p–n junction with the p-type CNT network. Through simple F2F assembly of the two blocks, three device types (CNT + CNT blocks, CeO₂/CNT + CeO₂/CNT blocks, and CNT + CeO₂/CNT blocks) are fabricated and systematically compared to elucidate the sensing mechanism and optimize the configuration. Among them, the asymmetric CNT + CeO₂/CNT device exhibits the most stable and sensitive response, reliably detecting NO₂ concentrations as low as 10 ppm even at room temperature and reaching a sensitivity of approximately 6.2 % at 120 °C for 30 ppm NO2. The superior sensing behavior arises from efficient charge transfer across the CeO₂/CNT interface and favorable adsorption-desorption kinetics provided by the high surface area of the CNT forest. These results verify the feasibility of compact, low-power NOx detection using a hybrid CeO₂/CNT structure for industrial and automotive sensing applications.