So, out porch light quit working a couple days ago. I figured the bulb had burned out, so I changed it. But it still didn’t work, even with a known good bulb.

I took off the switch plate, and pulled out the switch. It had power measured from ground (the box) to the input. But it also had power at the output, though a couple volts less. There was no voltage at the light. I took out the switch, and got the same result. 119 at the hot lead, and 117 at the wire that supposedly goes to the light. Any ideas what’s going on?

[Afternoon update]

So, I removed the light. With the wires disconnected at the switch, there is 119 between the hot lead and ground, and almost the same on the neutral. Neither of them should have any voltage on them. WTF?

[Saturday-morning update]

OK, here’s a new part of the mystery. The switch actually controls two lights (our main entrance is on the upper floor, and there’s another light on the ground floor below). When I get into the box where the switch is, there is a hot wire coming from the breaker, a triple wire with one of them going to the switch, and one going to each of the lights, and a pair of neutrals going to the two lights. But there is no neutral coming from the breaker.

And with the two neutrals connected and no switch, the hot, the ones going to the lights, and the neutrals all have voltage to ground.

Again, WTF?

Next step is to disconnect the two neutrals and see which of them or if both of them, are hot.

[Bumped]

[Afternoon update]

OK, I’ve exposed all the wires in the box. There is a black one presumably coming from the breaker, and it has 119V. There are two white ones, each going to a fixture, and they are both hot as well. There is no white one coming from the breaker. There are two yellow ones, presumably the hot ones, that went from the switch to the two fixtures. One of them is hot (the one going to the downstairs fixture). The other one, which goes to the upper fixture, is the only one in the box that isn’t hot. There is no ground wire; it’s grounded through armored conduit.

I continue to scratch my head. I guess the next step is to take the downstairs fixture off the wall and see what’s happening down there.

[Update a while later]

I’m busy with another project (installing new lighting in the kitchen), but I suspect that when I look behind the other fixture, I’ll see the neutral that comes from the breaker, which then feeds the upper fixture. And that’s probably where the problem is, though I have no idea what could have caused things to change.

[Update a while more later]

Per discussion of codes in comments, the house was built in 1977.