Abstract: In a motor vehicle heating or air conditioning device, a pollution sensor for detecting a pollutant, particularly carbon monoxide, in an airflow sent into the passenger compartment of the motor vehicle is arranged downstream of an air filter. The pollution sensor detects the presence of pollutant in the filtered airflow and supplies pollution measurements of the filtered airflow. The sensor is linked to a control circuit acting on an air intake flap to supply the passenger compartment with a mixture of external air from outside the passenger compartment and/or recirculated air from the passenger compartment, depending on the pollution measurement results.