The S-trap is no longer allowed
Years ago most plumbing codes banned S-trap configurations in favor of P-trap style fixtures. S-traps are no longer used in modern plumbing because on rare occasions with just the right conditions the water can be sucked out of the trap allowing sewer gas to enter your home. The risk from sewer gas can be much worse than just that terrible smell. Sewer gases can be poisonous or even explosive and are not to be treated lightly.
S-trap failure requires a large amount of water to flow quickly past the trap. This is usually caused by allowing a sink full of water to drain rapidly. The easy fix is to always refill the trap with water after using the sink. Plumbing codes, however, do not allow for common sense fixes, they are designed to make things work as predictably as possible.
Years ago most plumbing codes banned S-trap configurations in favor of P-trap style fixtures. S-trap fixtures are most often found in older homes and homes remodeled by do-it-yourselfers. A solution is to change the S-trap to a modified p-trap by extending the trap arm.
