Water from main floor shower drain coming out in basement bathroom sink.

[eskimos44]

Long story short... Going to remodel basement bathroom. Pulled out the sink and vanity. Disconnected the trap. Now when the shower is being used on the main floor, water is coming out of the trap pipe... What's going on here?

 

[frodo]

sounds like a partial obstruction in
the shower line, or. the sink is wet vented by the shower [illegal]