What is a cabin air filter? We all know that an air filter keeps dirt and grit from entering the engine, but what about the for air entering the inside of the car when you use your fan or a/c? The cabin air filter is similar to an engine's air filter in that by design it is to keep pollutants and pollen from entering the cabin while driving. Follow along as we show you how to replace the cabin air filter on our project GS300.