The answer to 'why is my P-trap foul smell' is that a smelly P-trap is usually caused by a dried-out or blocked trap that is allowing sewer gases to back up into your home. This can happen if the trap has been unused for a long time, or if there is a blockage or leak in the plumbing system.
Some common symptoms of a foul-smelling P-trap include a persistent sewer-like odor coming from the sink, bathtub, or floor drain. This can be a nuisance and may also indicate a larger problem with your plumbing. In most cases, the solution is fairly simple - you can try pouring water down the drain to re-fill the trap, or use a plunger or drain snake to clear any blockages. However, if the problem persists or you suspect a more serious issue, it's best to call in a professional plumber to diagnose and fix the problem.