The answer to 'pressure regulator foul smell' is that a foul odor coming from a pressure regulator is usually caused by bacterial growth or mineral buildup. This can happen when the regulator is not properly maintained or if there is a problem with the water supply. The most common symptoms are a rotten egg or sewage-like smell coming from the regulator or nearby plumbing fixtures.
In many cases, this issue can be resolved with a simple DIY cleaning or replacement of the regulator. However, if the problem persists or the smell is accompanied by other issues like low water pressure, it's best to call in a professional plumber to diagnose and fix the underlying cause.