The answer to 'backflow preventer foul smell' is that a foul odor coming from a backflow preventer is often caused by a buildup of bacteria or debris in the device. This can happen when the backflow preventer is not properly maintained or if there is a problem with the plumbing system. Symptoms may include a rotten egg or sewage-like smell near the backflow preventer.
To fix this issue, you may be able to do a DIY cleaning of the backflow preventer by following the manufacturer's instructions. This typically involves disassembling the device, cleaning the internal components, and reassembling it. However, if the problem persists or you are not comfortable working on the backflow preventer, it's best to call a professional plumber to inspect and service the device.