This is actually worse than it looks! If your car smells awful and you don’t know what it is, it just might be that you need to replace the vents or at least clean them. Before cleaning, always check for mold which is more troubling than a foul odor and can cause respiratory problems. If it is indeed mold, you must replace them. Otherwise, clean vigorously and use Q-tips to get into the crevices. More often than not, clogged vents are the cause of a check engine light.