The air purity sensor is part of Volkswagen’s “Climatronic” system in the Golf, Jetta, Touran, Passat, Touareg and Phaeton. Its task is to measure pollutants in the outside air that take the form of oxidizable or reducible gases. Oxidizable gases include carbon monoxide (CO), hydrocarbons (vapours from benzene or petrol) and other incompletely combusted fuel components. Nitrogen oxides (NOx) are reducible gases. If the quality of the outside air drops to a certain level, in a traffic jam or when driving through a tunnel for example, the control system activates the Climatronic’s air recirculation mode, thus preventing polluted outside air from reducing the quality of the air inside the vehicle.