The answer to 'my pressure regulator smells bad' is that a smelly pressure regulator is typically caused by a buildup of mineral deposits, bacteria, or other contaminants in the regulator. This can happen over time as water passes through the regulator, leaving behind residue that can create unpleasant odors. The smell may be accompanied by a decrease in water pressure or the regulator making unusual noises.
In many cases, you can try cleaning the pressure regulator yourself to resolve the issue. This involves turning off the water supply, disassembling the regulator, and thoroughly cleaning all the internal components. However, if the problem persists or you're not comfortable performing the repair, it's best to call a professional plumber to diagnose and fix the issue.