PURE cabin air filters are divided into two types: built-in and external. The built-in type is mostly located behind the passenger glove box and requires removing 5-8 screws to replace it; the external type is installed at the air inlet of the engine compartment, which is more convenient to replace but susceptible to moisture. Note that the airflow arrow must point toward the direction of the car during installation. Reverse installation will cause the filtration efficiency to drop by 60%. Some German cars adopt a dual filter element design, and the front and rear independent filter systems need to be replaced simultaneously.