The admittance value from most manufactures come apart , so you can fix , or replace , I have found in general plumbing , that issues come up and every time I have seen this happen , that taking apart [ some times cleaning ] you just reset by take apart & putting back together .
I do not see in the picture , so guessing behind the hole in wall .
I the diagram in the the next post it is not labeled - it is the Tee looking item next to the #1 - trap .
85 Excella , 31' , electric brakes , R. twin [ would like it to be a single ] , future upgrades , composting toilet [ replace black water tank with a 2nd fresh ] , solar power , instant hot water heater .
Tow 94 Dodge Ram , 4x4 , Cummins , AT .