Kitchen sink mystery part

[Tom S]

This black thing under the kitchen sink overflows when the dishwasher runs. What is it?

I don’t know how to look up how to clean it or fix it until I know what it’s called.

 

[MicEd69]

That is an air admittance valve, AAV. They are used when a vent to the atmosphere is not practical. Some places do not allow them as they can go bad. They are designed to let air in, so P-traps are not siphoned dry yet keep sewer gases from coming into the house. They do go bad at time as they have moving parts. Any plumbing or big box store or Amazon has them. Just make sure you get the same size. It is probably an 1 1/2", but it could be a 2".

 

[SHEPLMBR]

If it is spewing water, you probably have some kind of clog down stream.