Venting Issue

[pacer]

Whenever my second floor shower is used, the sink in the 1/2 bath below ( on main floor ) emits sewer gas .
The sink drains properly and I have confirmed that the p trap is IAW plumbing standards. In desperation, I have even installed an AAV in the sink plumbing but to no avail. I do not recall this issue being present when we purchased the house but cannot be 100% sure. I assume that something is blocking a vent line leading to the bathroom sink but the laundry tub and washer drain which I presume , share the same vent, do not have the same issue. Any thoughts ?

 

[CT-18]

I think you need someone in to have a look. You may have a hole in a vent line somewhere. They may need to do a smoke test or peppermint to find out what is going on. Your sure the trap isnt getting siphoned out when the bathroom above is draining.