When backflow preventers are installed or maintained, debris in the water line can travel into the valve body and get caught between the sealing disk and check seat. This is a very common situation and is referred to as "fouling" the checks.
A fouled check will not pass inspection. Many times the check simply needs to be removed and cleaned. In some cases, the rubber or seat may need to be replaced due to larger debris causing damage.
The best way to prevent the check valves from fouling is to make sure water lines are flushed properly before installation or before opening the upstream shut off valve after repair.
The Deringer dual action and true poppet check valves are designed to drastically reduce the chances of fouling. Our maintenance videos cover how to clean a fouled check without having to replace any components.