The answer to 'my pressure regulator smells bad' is that a foul odor from your pressure regulator is usually caused by a buildup of bacteria or debris inside the unit. This can happen over time as the regulator gets older and less efficient. The most common symptoms are a strong, unpleasant smell coming from the regulator itself.
In most cases, the solution is to replace the pressure regulator. This is usually a relatively simple DIY project, but it's important to follow safety precautions and turn off the main water supply before attempting any repairs. If you're not comfortable doing it yourself or the issue persists, it's best to call a professional plumber to diagnose and fix the problem.