Although this is a super well made (albeit pricey) tester, it has the same failing as most three prong testers. There are wiring errors that can't be detected by this - or any other three prong tester. Most of the time I use a Sure Test, and although the Sure Test can find some wiring errors the Hubbell can't, the Sure Test can be fooled, too - and it's not as durable as the Hubbell. If I'm suspicious of a receptacle, I have to get the Amprobe circuit analyzer out. If you need a super durable three prong tester, this is the one to get. If you need to analyze circuits, choke up the big bucks and get the Amprobe.12.3.17 Update - Hubbell tester failed shortly after the Amazon return window closed. I called Hubbell - and although they put up a bit of a fuss, they sent me a new one. Good to know they stand behind their products.