Help GFCI Tester Showing All Lights

This afternoon during an inspection when I tested an outlet all three lights lit up on my GFCI tester. There is nothing in the manual stating what type of connection this may be. Does anyone know what this means? I’ve never seen this before.

One would have to know what kind of tester you have. Most of the ones Ive ever seen or own have their own code chart attached. You did not say whether it is a 4 dollar tester or a $300 tester and which brand.

It is a combination problem.
Simple as that.
I assume it was fine on the others.
Try checking for ground and reverse with a multi meter.

This can happen if someone has a regular receptacle wired for 240V.

It is a cheap $9 GFCI tester from Home Depot. All of the other outlets tested okay. The attahed chart does not show the configuration for all three light coming on. I am at a loss on this one. Thanks for all of your replies.

Mark is correct. That’s the issue exactly.

I agree as well.

An improper MWBC with an open neutral and a load can do this.

I just noticed the other day on a new, 120V GFCI outlet, forget which brand, all three lights come on (cheap tester) while holding in on the GFCI reset button.

If your line voltage is on the high side an open ground can light all 3 on a cheap tester. (L/N and N/G lights end up in series). There will be one bright one and 2 dim.