The answer to 'backflow preventer foul smell' is that a foul odor coming from a backflow preventer is usually caused by a buildup of bacteria or debris in the device. This can happen when the backflow preventer isn't properly maintained or if there's a problem with the water supply. The most common symptoms are a rotten egg or sewage-like smell coming from the backflow preventer.
In many cases, you can fix this issue yourself by flushing out the backflow preventer. This involves turning off the water supply, disconnecting the device, and running water through it to clear out any blockages. You may also need to clean or replace the internal components. However, if the problem persists or you're not comfortable doing the work yourself, it's best to call a professional plumber to inspect and repair the backflow preventer.