Replaced a regular outlet with a GFI outlet. My 3 light tester shows an open neutral. I checked all my neutral wiring, all tight and tested all the outlets on that circuit. No problems elsewhere. I then put the old outlet back in place and tested, worked fine and the 3 light tester gave me the two yellow lights so that works fine. Could my gfi have an internal open neutral? Should I just buy and hook up a new gfi?