Sewer Gas Odor

99 percent of the time, sewer gas smells occur due to a lack of water in the trap. If you don't use a drain for two to three weeks, the water may simply evaporate. The purpose of a trap is to hold water to prevent sewer gas from entering a room. If you don't use that sink often, just be sure there is water in the trap.

If that is not the case, check for cracks or breaks in the drain piping. If you don't detect a leak, it could be cracked on top of the pipe.

Provided by: http://doityourself.com/drainage/sewergasodor.htm