A previous owner of my house (I swear it was not me :) ) appears to have broken off a screw in the hole (red circle) that allows the bottom of the outlet to attach to the box, leaving only the top hole (yellow circle) free to attach the outlet to. What's the best way to get the screw out? I know that one option is described in Removing broken screw from electrical box, but I'd like to preserve the existing threads if possible. I tried grabbing it with needle-nosed pliers from the front and twisting it out, but it appears stuck either via corrosion or because it's the wrong size (although they did get it in that far). I was thinking of trying the following approach:
apply WD-40 to the hole in the front and back to loosen up any rust
try grabbing it from behind the hole with curved needle-nosed pliers and see if I can at least move it out a little bit farther
if I can move it with the pliers, either unscrew it all the way, or get it out far enough that I could cut a groove in it to put a flathead screwdriver in
I could just try cutting the groove now, but that seems risky given that it might be stuck in there pretty well if it's the wrong size, and I don't currently have a lot of room for error if I cut it incorrectly.
What do you think? Thanks in advance for any help!
Corrosion isn't your problem. The screw was too long and it bottomed out on what looks like a cable clamp in the box. The screw is in compression. If you get in there with a Dremel with the tiny cutoff wheel, you should be able to saw through it. Once you do the compression will be gone and you should be able to finger-spin it out.
It looks like his mistake was using extra long screws because of the depth of the plaster there. And he went extra extra long, and was too long and bottomed out. Choose length more carefully, or use the feature on your wire stripper to cut bolts.
You'll probably have better luck with vise-grips than needle-nose pliers. After that, I'd hit it with something like liquid-wrench, specifically formulated to break up corrosion, or a penetrating oil. WD-40 isn't the best lubricant.
This would be tricky to do, but as a second option you could remove a little more drywall (to make space) and see if you can tighten a drill chuck around the screw. A hand drill has a lot of torque, and should break the seized threads free.
If you need to cut it you can but all you need to do is continue tightening it until it clears the hole. Or you can always drill it out any if you need to rethread the hole just find a larger screw and a tap and die kit. It's only a few threads I'd just use vice grips and force it through.