The Circuit Detective's Diagnostic Tree
Finding Your Open Hot or Open Neutral
If you have reached this page without beginning at the Start of the diagnostic tree, you may do better to start there.
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Since an open is an unintended discontinuity, locating it can involve experimentally disturbing connections till the bad one makes good contact again briefly or else deducing fruitful places to look for the discontinuity. There are two matters important to finding the bad connection. One is whether the hot or the neutral is open. The other is the extent of the outage compared to the extent of that circuit.
If a tester reveals Hotness still present at more than one dead item of the circuit (at the shorter slot of receptacles or the tongue in the bottom of a turned-on standard light socket, for instance), the neutral is open. When this is true, the other matter -- the extent of the outage compared to the circuit -- is easily determined because you can learn the whole circuit's extent when you turn off the breaker that is keeping hotness at the nonworking (dead) things. However, an open hot leaves you in the dark about what circuit you are dealing with -- unless you trust the labeling at the panel to be true and thorough. (In fact, your outage may extend to more than one circuit if one of the main wires is having trouble at your panel, the meter, or the power company's line; however, this usually shows itself in stranger symptoms than those that brought you to this page of the diagnostic tree).
If ten or more items are not working, you might want to look for the open first at the breakers and neutrals of your main panel itself. You would be looking for loose, corroded, or discolored connections of wires and testing for voltage at the breakers.
Next (assuming you don't have a generator transfer-switch that got knocked to OFF), look for the open out along the circuit. If you know the circuit's working things from its dead ones, but even if you have to guess that nearby live things may be part of the same circuit as your dead things, you can now position yourself among all the dead items you can discover. Face toward your main service panel. As you move in your imagination (without regard to walls or floors) toward the panel, the first working items (of the circuit, if you know it) that you would start to pass by are suspects, along with those dead items nearest to these working ones.
Now go plug a working, turned on lamp or nightlight into one of the dead outlets. Assign someone else the job of watching that light constantly and reporting to you immediately if it tries to light up -- even the slightest flash. [I suppose you could do it all yourself by dragging the light around with you on the end of an extension cord]. You are to go take the covers off all outlets and switches -- dead or alive -- along this dead/live border. (Yes, move furniture to discover outlets you never use or didn't remember existed).
Now plug something else into each outlet (dead or alive, along this border), one after another, and wiggle it side to side somewhat. Next go back to the same outlets and also to the switches and stick a strong, thin stick OF PLASTIC OR WOOD (e.g., chopstick) beside the device, pressing firmly on the wires you see, and then stick it more behind the device pressing and poking various wires you can't entirely see. You can even pound on the wall or ceiling near the boxes of this electrical border. The purpose of all these activities is to disrupt wires back into good contact. If at any time your helper tells you the light flashed or stayed on, stop where you are; this is where a connection needs to be improved. How to recognize the bad connection in an electrical box.
This method will often succeed, but certainly not always. So here's what else you can do. If it is the hot that is open, insert a non-contact voltage tester in past the sides of each dead device (receptacle or switch) so that it has chances to get up against each of the wires in the box. (You will probably have to loosen the device from the box somewhat to do this). If it stays lit or gives its alarm, then this is where the hot has lost good contact and where you will repair it. If no dead device shows this hot wire behind it, then it will probably be one of the working devices that you had exposed which hides the problem, and you will need to visually and manually check or improve the black-wire connections at these places. However, if it is the neutral that is open, you will probably have to resign yourself to checking and improving the white-wire connections at both the dead and the live places on this Dead/live border. How to recognize the bad connection in an electrical box.
More possibilities. Could you have overlooked items of this circuit that are out of the way: the doorbell transformer's box, wired-in smoke alarms, or junction boxes in attic or crawl space? Also it was common in homes built 1940-1970 to run a circuit through the light boxes more than the outlet and switch boxes, so there may be connections to be disturbed and checked there. Homes built 1900-1950 may still be relying on their original knob-and-tube wiring connections. If these were not soldered (well), they can be the cause of an open; they are not found in electrical boxes but in ceiling spaces and walls; if they are accessible, someone jostling them with a stick could make the light (in the method above) flash on.