Can the AAV malfunction and what are the signs?
Yes. Any AAV can malfunction or not operate properly because it is a mechanical device. Signs of that the Sure-Vent is not operating as intended, are foul odors. If you smell foul odors (sewer smell) near a plumbing fixture which has a Sure-Vent installed, this means the Sure-Vent is not closing properly to create a positive seal and should be replaced. Some individuals relate slow drains to the Sure-Vent not operating properly. The Sure-Vent is not the issue, the slow drain is related to a partial blockage within the drain itself. A partially blocked drain will fill with water when the fixture is draining, and the Sure-Vent recognizes the positive pressure from the restriction and stays in closed position to stop backflow out of the Sure-Vent. You should have your drain cleaned when this occurs.