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During a house sale, inspector pointed out the GFCI at the front of my garage was failing his tester. The trip light was always on but it still fed power.

Skip to the end if this is too wordy, but I wanted to cover everything I have verified so far.

While replacing it myself, I am flipping off breakers to find the one for it (key for later on) as I don't have all labeled on the panel yet. The new GFCI constantly trips. I remove the load wires and the GFCI is now good. The load runs to an outside outlet down the same stud gap which I course want protected as well. I fiddle with the outside receptacle and replace it as it was old and rusty. Nope, GFCI still trips. The outside receptacle has two lines and I can't tell which runs to the GFCI and which runs further down (or so I thought it was down) so I unhook one and try it. The GFCI and outlet now work. So what did I unhook?

Checking around the house, I find several kitchen/dining/deck lights not working. Not good.

Decide to bypass the GFCI and just pigtail the lines together. Now all the lights are working. About to give up for the night when I notice on the breaker box that I had left one of the others off from the initial flip testing. That one is labeled with kitchen & dining lights and GFCI.

After some more flipping with the unnoticed breaker, I find out that the line & load pairs to the GFCI were actually line & line, 120V on each when all breaker switches are on. So the outside receptacle was fine all along but it's not below the GFCI on either circuit.

That's not normal? What would be the purpose of doing that? I'll verify tomorrow, but it would also seem that both lines terminate at the GFCI. Apparently the old GFCI was able to handle this (somewhat) and the new one doesn't? There's no indication on the old one that it was double poled, it has the same line/load instructions on the back that every other one I have installed.

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Modern GFCI devices will not set if the LINE and LOAD are reversed. If there's power on the LOAD wires, the device will not (and should not) set. It sounds like the GFCI is working as designed.

You're going to have to figure out how both circuits are wired, to determine if this is intentional or accidental. If it's intentional, you can simply cap one set of wires in the box. If it's accidental, you'll have to disconnect the crossed wiring, and then connect the LOAD wires to the GFCI.

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