My friend lives in a 100 year old row home that has had several renovations by several different people.
His kitchen was redone a couple of years ago with new wiring and fixtures. He tells me everything was working fine until a couple of months ago. As far as he knows, nothing was changed. He has had other work done in the house but he says that is not when the problems began.
The problem is with two kitchen lights that operate on two separate switches (in one box) fed from the same circuit breaker. He says they used to operate independantly as expected but now turning on EITHER switch turns on BOTH lights. When you turn on either switch the other switch has no effect on either light. So basically either switch turns both on and both switches have to be off to turn both lights off.
I suspect one or both switches are bad and just need to be replaced but just in case here is more detail. I took off the switch plate and have tried to diagram what I saw (attached), but I did not remove the light covers so I'm only speculating how they are wired. The important point is that they WERE working properly (so he says - and I KNOW he would not mess with wiring).
The feed from the panel is 12/2 romex (B&W + ground) and coming into the switch box from the lights is 12/3 romex (B,W,R + ground). You will notice a bit of white on the red wire. The installer put a piece of white tape on it for reasons I can't figure unless that's how he was trying to mark it as hot. The switch screws are wrapped with white tape so maybe that's just what he used though I would think it should be black tape?
I didn't have any spare switches and it was late but do you think that's the first thing to try? Any other suggestions?