That is right, the lights usually tell you if there is a problem and gives a basic idea as to when something is reversed or not hooked up. We have caught a few mistakes here and there with those, and depend on them after a trim, to hot check out the house before we leave so we dont have to get a call 'this isnt working' or 'that isnt working' because it was when we tested it. Also, the 'wiggy' works well, for those who would rather not use one of those testers.
