Under normal circumstances, most air filters are very different from the positive and negative sides or adopt a special-shaped design. If the reverse can not be loaded into the air filter shell, there will be no reverse loading. However, some products with a plastic frame may be mounted upside down, and the frame may cause the maintenance technician to misjudge the installation direction. In addition, if the air filter is installed vertically, the Angle of the air filter shell can be opened is relatively small, which makes it difficult to observe and confirm the installation direction of the filter element, and it is easier to install the wrong filter. In addition, in order to save time, some maintenance personnel neglect to identify the direction when loading the new air filter, so that the filter element is installed in reverse.