The cars air filter is usually a pleated-paper filter that is installed in the vehicle's passenger compartment's outside air intake. Some of these filters are rectangular and resemble the engine air filter in design. Others are specially designed to fit the space allowed in specific cars' outside-air intakes. Although it may not seem like a vital part to check and replace on a regular basis, air filters are crucial to preserving your car's performance. The filter stops tiny particles from getting inside the engine and potentially causing costly avoid damage.