The air filter is very simple to change in the C30. First, open the hood and locate the air box. This is in front of the engine in the center. It has two phillips head screws holding the filter tray in the air box. Unscrew the screws and lift out the filter tray. Remove the old filter and insert a new one, being sure to press it all the way in. Reinstall the filter tray, being sure the seal along the top edge is still in place and resecure with the screws.
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