I am having the following problem. A few months ago I finished my basement (1 Bedroom and 1 Bathroom). I had an existing GFCI outlet down there to which I connected a couple more outlets, an exhaust fan in the bathroom and some LED lighting. Everything worked just fine until 2 days ago. Nothing works anymore in the basement. Breaker did not flip. I disconnected everything that I had connected to this one existing GFCI outlet when I finished the basement. I also took off the existing GFCI outlet to have better access to the incoming cable (source). Below you see the existing circuit (as far as I was able to find out) as well as the measured voltages in the finished basement. Like I said, I disconnected everything in the basement (back to original condition before finishing the basement) and still getting this low voltage. Too low to make anything work. Why did that happen all the sudden and what can I do to fix it? Thanks a lot!
You picture shows a T where the wire goes to the outlet and garage. Is it possible that that is in a box somewhere and the bad connection is there? Look around. Maybe its in the ceiling of the basement.
Were the "backstab" (the push in connectors on the back of outlets) used? I have seen those become intermittent.
Or does the circuit to from the upstairs bathroom to the to the garage and then from the garage to the basement?