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We present a microelectromechanical systems-based sensor for the thermal detection of changes of gas mixtures such as the CO2 concentration in air that is of interest in air conditioning climate control within buildings. Key properties of the system are low power consumption (<; 10 mW) and high long-term stability through the absence of moving or consumptive components. The used sensor chip has three silicon-microwires (thermistors) surrounded by the gas mixture to be analyzed. A centered wire (heater) is supplied with sinusoidal heating power. This induces a thermal response via the surrounding gas to measurement wires (detectors) located in different distances from the heater. The phase shift between heating power and induced thermal responses at the detectors is analyzed and depends on the thermal properties of the gas. After calibration, the sensor is able to quantify the concentration of an individual component within a mixture of different but known gas components. This is demonstrated by measuring the CO2 concentration in N2/CO2 mixtures with a resolution of 0.2% at constant pressure and temperature.