PEX is the most commonly used material for water distribution systems in homes today; it is generally accepted as a good and reliable material. However, as with polyethylene, PVC, and even copper, there were problems with the materials during the first few years of use. Most contractors use metallic fittings to connect the PEX water lines. Some manufacturers made these fittings with zinc content that is too high. The problem is that with certain water conditions, the zinc would leach out of the fittings causing two problems. First, the walls of the fitting become very thin, and eventually begin to slowly leak. Second, the deposits from the process can clog fixtures, valves, etc, reducing water flow and/or pressure. At least one manufacturer of these fittings has admitted fault and submitted to a class action lawsuit. Their fittings may be stamped with "F1807" on them. However, identifying exactly which fittings/manufacturers may have the problem is not that easy as details are still developing. Most of the problems have been found in homes constructed between 1996 to 2010.
Our suggestion is that if your home inspector determines that defective fittings may be present in the home, based on visible clues, part numbers, manufacture, date of construction, etc. you should not take the suggestion for further investigation by a licensed plumbing contractor lightly. They are the ones that could be expected to be the most informed about the specific materials used in that particular home.
More information is available on the internet at various web sites including: Zurn Lawsuit