Help! Our gas stove top is letting in enormous amounts of cold air through the exhaust located on the top of the stove. Is there a way to block this cold air? Right now we are covering the exhaust with towels which is not safe or cosmetically acceptable.

Help! Our gas stove top is letting in enormous amounts of cold air through the exhaust located on the top of the stove. Is there a way to block this cold air? Right now we are covering the exhaust with towels which is not safe or cosmetically acceptable.
The back flow preventer flapper may be stuck open(up) or you don't have one