I have seen it where I would test a GFI with my little tester, then I would go to rest it and the "Open Ground" light would come on and it looked like a bad GFI. I would then trip it again with the GFI's tester button, and reset it once more. Then averything would act normal. I have seen these several times in the past year in many different house. It seems you have to test and restet twice before it resets properly. Just my 2 cents. By the way,, I have seen a gfi, tried to test it with my tester and nothing happened. I would then check the test button and it would work. So My gFI tester went bad. So now I carry 2 GFI testers just in case.
