Normally when you see this, the first thing that you will want to check is all of the connections on the back of the receptacle, and if all of those are tight, then check the rest of the receptacles in the circuit.
– Many times when you see this, there is a loose neutral connection on the receptacle.
– You will also want to make sure that you do not have any other appliances or electronic devices plugged in on the circuit. Sometimes components inside of these products can create feedback or leakage to another wire that will give these types of testers problems.
– Plugging into a 12V DC to 120V AC converter will also have this issue.
If none of these steps resolve the issue, then we recommend that a licensed professional examine the situation for you.
Gardner Bender FAQ 
I found this answered question to be the most helpful thus far in my quest to figure out why all 3 lights are on when I plug in the oulet tester. I understand that this post is nearly 2 years old but was wondering if you could elaborate more on why this result would be preset when a 12v dc to 120v ac converter is connected to the circuit.
Hello,
The design of these testers is to function off of a normal 120 VAC circuit. When you add different variables like the one you are describing, it confuses the tester. This isn’t something that we have been testing for either and it’s nice to see the different applications people use these testers for. I’ll pass your situation on to our engineering team so that they can test this scenario in the future.
Thank you,
GB Tech Support
It could also be that the voltage is wrong on that line. I’ve seen this happen if the line is running 230 volts instead of 120.