Aftee tripping a gfci and resetting my tester showed only 1 light on. After a few seconds it came back on. Does anybody know what that means? Loose connection?
Check with another tester. The 3-prong testers will have loose internal connections once you jam them into enough receptacles. I always carry multiple to double check if I get an off reading. Throw away ones you think are compromised.
Wiggle the tester in the receptacle. If the light goes on/off occasionally, the contacts in the receptacle may be old and have lost ability to hold in the plug.
Trip and reset the GFCI a couple more times. If the lights stay, then there may actually be a loose connection in the receptacle, at the wire connections, or somewhere upstream and you happen to knock it loose a little when tripping the GFCI that likely has not been tripped in a long time.
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I just tested it with another one I have and it seems to be true but I may go ahead and replace preemptively. It’s only about 3 months old
I’ve gotten all kinds of crazy readings momentarily after testing/resetting GFIs with those 3-prong units. I still carry a good old fashioned “wiggie” to “test my tester”. I agree with what Yu says also. Those testers do fail after a while. In my experience they start reading open grounds falsely. They’re so cheap that I try to just replace them every year or so. Usually, I drop one behind a washing machine long before that!
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This one is not that old but I’ll probably buy a new one today. Thanks
Buy two or three. 
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As already pointed out 3 light testers are not the most reliable testers out there. GFCIs also have a fairly well accepted lifespan of about 25 years.
If you are going to diagnose something it helps to have a repeatable failure (intermittents are a PIA). You don’t indicate if this happened multiple times with the same outlet or if perhaps it happened with other outlets as well. The latter would surely point towards the tester rather than the outlets. In most cases I would only have the one three light tester, but the home very probably has multiple GFCIs. On the other hand I do carry alternate electrical testers (wiggy). When in doubt a basic solenoid tester is the tool you want to fall back on.
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Did you trip it with the tester or the test button on the receptacle?
Did you try it the other way to see if the problem happened regardless of method of tripping?
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Next time, grab a lamp and see if the outlet still works. If the neutral is indeed open, it won’t. This is possible given the design of a GFCI outlet which involves an electromagnet and a relay: it’s possible one contact is bad.
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The tester says open neutral. This happens when the gfi trips.
I think that he said that he reset the receptacle that’s why the green indicator light is on in both photos.
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This has happend to me several times, using different 3-light testers. I’ll wiggle the tester just a little or else unplug it and plug it back in and it’ll give me the correct reading.
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Same here, Christopher.
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I did repeat it a few times. I also went home and tested my receptacles agaisnt another tester. This, coupled with the fact that my tester showed no signs of issues at any other receptacle at that home leads me to believe it was the receptacle and not the tester. I definitely plan on carrying more than one going forwards.
Also, this home had ungrounded receptacles except in the kitchen and bathroom. I know the last time I had a bad tester it showed 100% ungrounded. This one is only 3 months old or so.