A CO2 sensor based on an AlGaN/GaN high electron mobility transistor (HEMT) was developed using ZnO nanorods as the sensing material. The sensor showed a reliable response to a wide range of CO2 concentrations from 500 to 100000 ppm at 300 °C. The CO2 response of the device was tested from 25 to 400 °C, and the sensor started to exhibit responsivity to 30000 ppm CO2 gas at 150 °C. The responsivity increased with the ambient temperature until the temperature reached 300 °C, and it decreased from 350 to 400 °C. The maximum responsivity of the sensor with ZnO nanorods was 4.31% for 10% CO2 exposure at 300 °C. In addition, the effect of humidity on the CO2 sensing characteristics was investigated. AlGaN/GaN-heterostructure-based CO2 sensors functionalized with ZnO nanorods have high potential for applications in the chemical, medical, energy, and food industries.