The answer to 'my drain trap smells bad' is that a smelly drain trap is usually caused by a dry trap or a clogged drain. The drain trap is the curved pipe underneath your sink or floor drain that holds a small amount of water to prevent sewer gases from entering your home. If this water evaporates or the trap becomes clogged with debris, it can allow unpleasant odors to come up through the drain.
Common symptoms of a smelly drain trap include a persistent sewage-like odor coming from the drain, sometimes accompanied by a gurgling sound. To fix the issue, you can try pouring a cup of water down the drain to re-fill the trap, or use a plunger or drain snake to clear any clogs. However, if the problem persists, you may need to call a professional plumber to inspect and repair the drain system.