The answer to 'my P-trap smells bad' is that a smelly P-trap is usually caused by a dry or blocked trap. The P-trap is the curved pipe under your sink that holds water to prevent sewer gases from entering your home. When the water in the trap evaporates or the trap becomes clogged, it allows unpleasant odors to come back up through the drain.
Some common causes of a smelly P-trap include infrequent use of the sink, a leak in the trap, or buildup of debris and grime. The solution is usually simple - just pour some water down the drain to refill the trap, or use a plunger or drain snake to clear any blockages. However, if the problem persists, it may indicate a more serious issue that requires the help of a professional plumber.