The answer to 'my shut-off valve smells bad' is that this is a common issue often caused by bacteria or mold buildup in the valve or nearby plumbing. The smell can indicate a problem with the shut-off valve itself or a larger issue in your plumbing system. It's important to address the source of the odor to prevent further damage or health concerns.
Some common causes of a smelly shut-off valve include water stagnation, mineral buildup, or a small leak allowing bacteria or mold to grow. The smell may be musty, rotten, or like sewage. If the odor is strong or persistent, it's best to have a professional plumber inspect the valve and surrounding pipes to diagnose and resolve the issue.