Mobile Home Drains
Venting

Venting is one of the areas where mobile homes differ from site built homes.  In site built homes every drain has a vent pipe that eventually goes up through the roof.  The  purpose is to prevent the siphon effect of water going down the drain from pulling all the water out of the trap.  In mobile homes venting is frequently accomplished with one way air valves that will be located under the sink.  

If the valve fails you may smell sewer gas inside the home because it is no longer blocked by water in the trap.  The valve may be threaded and easily unscrewed for replacement or glued on and you will have to cut the line to replace the valve.