The maintenance-free, multifunctional duct air quality sensor measures CO2 content and mixed gas and additionally temperature and humidity (relative, absolute, enthalpy, dew point). The devices with relay output for a 2-point controller or a 2-stage 2-point controller and color-changing LCD display allow a wide range of applications. Depending on the model, the sensor can be individually configured via Thermokon USEapp. Tool-free opening, closing and wiring as well as removable cable entries ensure quick and easy installation.