The answer to 'my plumbing repair smells bad' is that there are a few common causes for this issue, including a dry p-trap, a clogged drain, or improper venting. The smell is typically caused by sewer gas escaping from the plumbing system into your home.
If you've recently completed a plumbing repair, it's important to ensure the work was done properly and that all connections are sealed. A dry p-trap, which is a U-shaped pipe under a sink or drain, can allow sewer gas to come back up into the home. You can fix this by running water down the drain to refill the trap. A clogged drain can also cause smells, so you may need to use a plunger or drain snake to clear any blockages. Improper venting, where the plumbing system isn't allowing sewer gases to properly vent outside, is another common cause that will require a professional plumber to diagnose and repair.