The answer to 'why is my pressure regulator foul smell' is that a foul odor coming from your pressure regulator is usually caused by bacterial growth or debris buildup. This can happen if the regulator is not properly maintained or if there is an issue with the water supply.
Some common symptoms include a rotten egg or sewage-like smell, discolored water, and reduced water pressure. To fix this, you'll need to clean or replace the pressure regulator. This is generally a DIY-friendly task, but if you're not comfortable working on plumbing equipment, it's best to call a professional plumber to diagnose and resolve the issue.