The answer to 'pressure regulator foul smell' is that a foul odor coming from a pressure regulator is usually caused by a buildup of bacteria or mold growth. This can happen when the regulator is not properly maintained or if there is a leak that allows moisture to accumulate.
Some common symptoms include a rotten egg or sewage-like smell, discoloration of the regulator, and potentially visible mold or slime. To resolve this issue, you'll need to clean and disinfect the regulator. This is generally a DIY-friendly task, but if the problem persists or you're uncomfortable working on the plumbing, it's best to call in a professional plumber.