The answer to 'my P-trap smells bad' is that a smelly P-trap is often caused by a buildup of debris, mold, or bacteria in the trap. This can happen if the trap doesn't get flushed regularly or if there's a leak that allows sewer gases to escape. The symptoms are a foul odor coming from the sink, tub, or floor drain.
To fix a smelly P-trap, you can try flushing it by running hot water down the drain for a few minutes. You can also use a plunger or small brush to dislodge any buildup. If the smell persists, you may need to disassemble the trap and clean it thoroughly. However, if the issue is caused by a more serious problem like a cracked or damaged trap, it's best to call a professional plumber to diagnose and repair the issue.