Offensive odors, dirt or debris coming from your sink's drain will alert you of a problem with the sink's drain pipes. The problem could be as simple as a clog near the top of the drain pipe system or it could be due to a clog in the plumbing's vent pipes.
Clogs form in your sink's drain pipes as you wash debris down the drain. Small materials such as soap, hair and dirt from your skin will wash down the sink's drain along with the wastewater. Some of these materials will gather on the walls of the drain pipe, accumulating slowly over time. Eventually a clog in the drain pipe will block the pipes. A clog near the sink's drain opening will allow clog material to wash up into the sink and will emit a bad odor from bacteria growing in the clog material.
Remove the sink's stopper or drain cover. The clog material may be attached at least partially to the stopper or drain cover, which you wipe off using a rag. Insert a needle nose pliers' jaws into the sink's drain opening and pull out any clog material you can grab. Use a coat hanger as well to fish the clog material out of the drain. Plunge the drain if necessary to force the rest of the clog down the drain pipes.
Vent Pipe Function
The vent pipes help keep sewer gasses from escaping up into the drains in your home, venting the gasses out of the vent pipe openings above the house's roof. A clog in the vent pipes will keep sewer gasses from escaping safely, pushing them out of the sink's drain instead. The clog in the vent pipe may also affect the water level in the sink's p-trap, or the curving section of pipe that stays full of water. With the p-trap partially drained, the sewer gasses pass through uninhibited.
Clearing Vent Pipes
You must use a ladder to get onto your house's roof so you can check for clogs in the vent pipes. Shine a flashlight down the pipes and look for any debris inside. Feeding a drain snake's flexible line down the vent pipes may dislodge the clogs. If you cannot reach the clogs with the snake or if you do not have a drain snake, use a garden hose to spray water down the vent pipes to dislodge clogs.