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1) Double check and confirm that the LINE and LOAD wires are not reversed on the GFCI. The circuit feed hot and neutral wires need to land on the LINE side of the GFCI receptacle. If you have a 2 wire lead AC voltage tester or an AC voltmeter available, this can easily confirm the LINE side connections.
Thanks for the replies.
1) Does the circuit contain any splices in junction boxes along the way or was it installed as a "home run" from the panel to the receptacle box?
2) Do you have a 2 wire lead AC voltage tester (a contact type) or better yet, an AC voltmeter to confirm the voltages at the circuit breaker and also at the GFCI LINE side?
1) Did you plug anything into the GFCI? Does it work and can reset and test properly? Are you using the cube tester just to confirm the wiring at the GFCI?
2) Possible that the 3 prong cube tester is defective or is providing a false reading? Thus the reason to always have an AC voltmeter readily available whenever performing electrical work.
3) Since you temporarily installed a regular receptacle to the breaker and that works, then the breaker is not the problem. Either a defective cube tester or a defective GFCI.
Plug the cube tester into another known working receptacle and confirm if the tester is showing OK on that receptacle.