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When I plugging coffee machine in the GFCI outlet, it works but does not in the downstream outlets. The Voltage of downstream outlet drops to 87V after plugging in the downstream outlet.And shows open neutral if plugging tester. GFCI wasn’t tripped and voltage was still 120V. Test-reset GFCI will bring the Voltage back. Any suggestions to troubleshoot?

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    As @Barry notes below, its sounding like you have a bad connection at the GFCI or downstream. Backstabs (if used) are a "feature" that should not be used. Please ensure all wiring is sound, no cuts, breaks or otherwise. Make sure your wirenuts are tight and properly installed. You may even have to replace the outlet. CHECK BOTH ENDS *(the GFCI as well as downstream) – noybman Dec 29 '19 at 1:37
  • @noybman I can follow your comment better than the answer Barry provided. I agree it is a loose connection that has a problem under load. I also have a love / hate for backstabs, they have made me tens of thousands over many decades. I think they should be illegal but are “safe” when installed to code. I can give your comment an upvote + and the question as it is well defined+ – Ed Beal Dec 29 '19 at 4:02
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Only do the following if you are familiar with electric wiring and the necessary safety precautions.

  • Switch off the circuit breaker that controls the outlet with the GFCI.
  • Open the outlet and remove the GFCI.
  • Check the wiring.

There should be two wires (black and white) going to the terminals marked LINE. There should also be two wires (should be black and white) connected to the LOAD terminals. The hot wires are black and the neutral wires are white.

If these check out, and if you think you are prepared, then switch the breaker back on and check the voltage at the LINE terminals (should be about 120 V).

Then check the voltage at the LOAD terminals (should be the same voltage).

Push the TEST switch on the GFCI. The voltage at the LOAD terminals should go to 0 V while the voltage at the LINE terminals should stay at 120 V.

If all of this checks out, you need to check at least one of the downstream outlets. Again, they should have a black and white wire connected to their terminals and the voltage should be 120 V.

Trip the GFCI and check that the voltage goes to 0 V.

Note that if any of the connections use the backstab (located at the back of the outlet), you should change those to connect with the screw terminals on the outlet. Backstab connections are known to be unreliable.

If everything checks out, then you may need to call an electrician.

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    A suggestion to help this answer get the vote it deserves, please break this into some bullet points or at least paragraph separated as it is a bit rough to follow right now. I'd personally suggest he not apply breaker power until each outlet is checked from A-->b-->C etc. and ideally, a problem has been found and fixed. – noybman Dec 29 '19 at 1:45
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    Don't put spaces in the fronts of your lines, it causes weird formatting. You also need two newlines (a blank line) if you want a paragraph separation. Please take the time to learn SE edit formatting; there are help links in the edit window. If you are copying content from elsewhere and it's coming in this way, don't copy content except in attribution. – Harper - Reinstate Monica Dec 29 '19 at 4:20
  • @Harper Thanks for the tips. – Barry Dec 29 '19 at 13:10

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