Carbon dioxide (CO2) sensors are often deployed in commercial buildings to obtain CO2 data that are used to automatically modulate rates of outdoor air supply. The goal is to keep ventilation rates at or above code requirements, but to also to save energy by avoiding over-ventilation relative to code requirements. However, there have been many anecdotal reports of poor CO2 sensor performance in actual commercial building applications. This study evaluated the accuracy of 44 CO2 sensors located in nine commercial buildings to determine if CO2 sensor performance, in practice, is generally acceptable or problematic. CO2 measurement errors varied widely and were sometimes hundreds of parts per million. Despite its small size, this study provides a strong indication that the accuracy of CO2 sensors used in commercial buildings is frequently less than is needed to measure peak indoor-outdoor CO2 concentration differences with less than a 20% error. Thus, we conclude that there is a need for more accurate CO2 sensors and/or better sensor maintenance or calibration procedures.