How to debug a light fixture that is no longer working

Now that I know what the problem was, I'm going to try to write up a more general guideline for complete amateurs like myself who find themselves in a similar situation. Let me begin by stating the obvious, though: if the work is more complicated than replacing a switch or an outlet, you probably need a professional.

Also, always flip the circuit breaker to cut off power before accessing any live circuits.

OK, to recap the issue: after replacing some ugly but working light switches in my bathroom, the vanity light fixture mysteriously stopped working. It was clear that something I had done caused the problem, but I had no idea what. I need to trace the issue to the source.

Step 1: replace the new switch with one I know is working to make sure the new switch isn't the problem. (It wasn't.)

Step 2: replace the fixture with one that is definitely working to make sure the fixture isn't the problem. (I bought a bare socket for $2 to test with. This wasn't the problem either.)

Step 3: if the fixture and the switch are both fine, then the problem is likely the wiring. It's helpful at this point to step back and understand how the wiring in the room fits together and what the wire colors mean. Let's start with the easy ones:

• Black: hot or live wire, bringing power from the power source to the outlet or switch
• White: neutral wire
• Green / bare: ground wire

Now the trickier ones:

• Red: these are also live wires. They can be used in a few circumstances, but in my case they indicated wires bringing power from the switch to a fixture.
• Blue: another live wire, in my case indicating live wires pulled through conduit.

So the general flow of power in my bathroom looks like this:

• Black -> outlet -> white
• Blue -> switch -> red -> fixture -> white

Now also understand that switches and outlets in a room are usually connected serially. Knowing this, you can figure out the flow of power in a room.

In my case, I used a non-contact voltage tester to see which wires were actually live. In other words, I just touched this pen-shaped thing to each of the hot -- red, blue, or black -- wires in the chain leading to the fixture to see which ones actually carried a charge. And it turned out that none of them did. Not even the blue wire leading into the switch was live. (Side note: if I had the non-contact voltage tester to begin with, I could have saved a ton of time swapping out switches and fixtures, skipping steps 1 and 2 above.)

Once I realized there was no power coming into the switch, my focus shifted one step upstream in the chain to a fourth electric box on the wall that contained the switches for the bathroom fan and overhead light:

It hadn't occurred to me that these two switches, which were both working fine after I replaced them, would have anything to do with an unrelated fixture. But sure enough, when I cracked the box back open, I spotted the yellow wire nut connecting the three blue wires.

That is, now it is connecting the three blue wires. At the time, the third blue wire, the one leading to the downstream vanity switch, was disconnected, because I had forgotten about it and forgotten that the hot wires are connected serially.

Et voila.