If you have an ohm meter you can check the pins of the extension cable from end to end. Check for any pins with unusually high restiances. You can also check for shorts from pin to pin. Radio Shack has ohm meters for $30 and they have a 30day return policy. I have had extension cables give horizontal lines that slowly foat down the screen and other wierd noise problems, but they have never kept my screen from working. Opens or shorts are the only things that could make a vga extension cable not work.