Carbon nanotubes (CNTs) are extremely sensitive to the electronic perturbations from environmental gases. However, the electrical detection of carbon monoxide (CO) is still a challenge. Here we show that 1 ppm of CO can be electrically detected at room temperature using a CNT resistor and the detection is proposed related to the sidewall functionalities (COOH). A simple all-CNT electronic nose composed of two resistors (or one resistor plus one transistor) is able to differentiate CO and other oxidative gases (for example NO and NO2). The electronic nose is working on both conventional SiO2/Si and flexible substrates.