The answer to 'my backflow preventer smells bad' is that a foul odor coming from your backflow preventer is typically caused by a buildup of bacteria, mold, or other organic matter in the device. This can happen if the backflow preventer is not properly maintained or if there is a problem with the water supply.
Some common symptoms of a smelly backflow preventer include a rotten egg or sewage-like odor, discolored water, and potentially even visible growth of mold or algae. The best solution is to have a professional plumber inspect and clean the backflow preventer to remove any buildup and disinfect the device.