There has been a sort of survey conducted recently that reports a very large percentage of non-working GFCIs are in the field. It is suggested that many are damaged by exposure to weather and power surges.
I have yet to see a non-working GFCI for any reason other than reversed line and load.
But that is because there are so few of them anyway.
I know that even I don't test my GFCIs monthly. In fact I just recently got my sons to test them all upstairs. I think I have one in the basement that hasn't been tested for a year and a half.
This may be one that falls into that category.
Perhaps so. Silly me, when I read that report I thought it said that it was wired in