Originally Posted by Donedroolin
So you mean top of vent under sink. Is it a carbon filter of some sort. And thank you
Yes, the top vent under the sink. It's a one way valve that allows air to enter the drain line so water won't be siphoned from the sink trap when draining the sink but blocks gasses from the holding tank entering the room (when it is working; they are subject to failure). If that valve fails in the open position, it will allow gasses from the holding tank to escape into the room. If it fails in the closed position (rare, btw), water will be siphoned from the trap and gasses can escape into the room through the sink drain. Any home center or hardware store should have AAVs although they may refer to it by a regional slang or brand name. Take the old one with you when you go looking for a replacement.