I thought I know a lot about electricity (graduated from Engineering university). I am wrong when working on home electricity. This little and inexpensive gadget is great. It helped me to detect any outlet that is not wired properly. Before this I test outlets, including the ones I installed, by plugging a small light bulb. If it lights up, the outlet is good. WRONG. This gadget reveals outlet wiring issues even if the plugged in lamp turns on: 1) neutral and hot swapped (one in my garage is this way, I fixed this) 2) open ground on a new outlet I installed even with correct wiring but outlets down same line are OK. Turn out this outlet is defective, replaced with a good one and the issue goes away/. 3) GFCI compliant test: hard to do without this gadget .... and many more. I noticed the "open ground indicator" is extremely confusing : supposed to light up only the center YELLOW light. Mine also shows little bit of RED and half lighted right YELLOW !!! One can confuse this with "CORRECT". I need to buy a different brand to try out. Downgraded from 5 to 3 stars due to this issue.