enter image description hereWe have a Honeywell programmable light switch at the front door. The light switch can be programmed for days of the week; it's set to go on at 5pm, off at 7am, every day.

The other day the light fails. I try turning the light on and off. That doesn't work. I assume it's the bulb, so I replace it. Doesn't work. I try another known bulb: no go. Doesn't work.

So then I'm thinking the switch itself is bad. But there are three weird things:

  1. When I'm screwing in the new bulbs, the bulbs flicker on for just a split second as they're going in. This isn't a matter of position or angle or anything. They just flick on and then off.
  2. In the same way, the switch itself has a little on/off button. And when I try to reset it by turning it off and on, the light bulb flickers, just for a second.
  3. The programmable switch works with a little LCD display on it -- like on a Casio watch. And the display is now always off - regardless of whether there's a bulb installed. I think there's some way with these kinds of switches that the switch needs a completed circuit through the bulb in order to power the switch display (or something like that), but I definitely don't understand how that works.

All of this is to say: Am I still right to assume the switch itself is bad? Or could it be the socket up there on the ceiling? Obviously, the socket issue, since it's inside an outside fixture, is more of a pain in the neck.

Thanks in advance!

OK. Added a picture of how it was wired. I believe this to be original 1920s knob and tube wiring. Wondering what options I have for a switch replacement in this case, either with a programmable or even a smart/wifi outlet.

  • What model is said switch, and can you post photos of the inside of the switch box involved please? Jan 30 at 1:40
  • This sounds like a bad connection could be the cause. I would be checking for a backstabbed connection , loose wire nut or broken wire, it could be the hot or neutral. Since your display is out check the timer first (dose the timer have a battery? ) I use a different brand intermatic and they use the battery for the display on the st01 model I use but the timer stays in the last position until the battery is replaced. So it could be a bad connection or a battery issue if similar.
    – Ed Beal
    Jan 30 at 16:42
  • Switch is Honeywell RPLS530A. I checked connections to the switch; didn't solve it. Then I installed a regular single-pole switch that I had lying around, and that DID solve the problem. This means the light socket wiring is fine and the old switch is bad, right? Weird thing: it's a 3-way wall switch, installed in a non-3-way, application. The 3 wires on the back labeled C, 1, and 2. The 1 and 2 wires connected to what I gather is the hot wire, and the C wire connected to the neutral. Anyway, what to do next? Do I need to find a different replacement switch? There's no ground wire...
    – TK in CA
    Jan 31 at 22:14
  • Bonus amateur question: In my knob-and-tube wiring back there, is there a simple way for me to know which is my hot and which is my neutral? The replacement switches I'm looking at mostly say "access to the neutral required," and it occurs to me that I'm not certain which is which.
    – TK in CA
    Jan 31 at 22:22
  • What type of bulb was the old bulb, and what type of bulb is the new bulb? (incandescent, CFL, LED) Jan 31 at 22:52

Your switch is wired up wrong

The RPLS530 does not require neutral, according to its instructions; however, yours appears to be incorrectly wired. According to the instructions, C and 2 need to be connected to the incoming hot in a single-pole setup like yours, while 1 connects to the switched-hot off to the fixture. I would expect the switch to function incorrectly in your current wiring (with 1 and 2 to always-hot and C to switched-hot); however, it does not explain your current behavior, so I would suspect the switch has failed, based on the troubleshooting you've done so far.

It appears that you have no neutral at the box

Furthermore, it appears that you have no neutral at your switch box, since I see no wires spliced through in the back of the box in your picture. This means that you'll want to go with another RPLS530A, or a similar type that does not require neutral, as your replacement switch. You'll need to obey the device's minimum load rating when you do, though; for the RPLS530A, you need to provide at least 40W of load for the switch to power up properly.

  • Thanks so much! So would you describe my wiring as having one hot wire (on the left) and one load wire (switched hot) with no neutral or ground? (I ask because a bunch of the switches require a neutral, and you seem to be saying that I don't have one. But most of what I've read about knob and tube refers to them as hot/neutral systems) Thanks again!
    – TK in CA
    Jan 31 at 23:48
  • @TKinCA -- yeah, electricians back then didn't bother bringing neutral to switch boxes Feb 1 at 0:04
  • Thanks @ThreePhaseEel Just to make sure, I tried re-wiring the "failed" switch properly, but it still didn't work. Are there any models that don't require a neutral and also can accommodate <40W loads? Looks like the Lutron Caseta might be one of them, but don't have any of their other products installed here.
    – TK in CA
    Feb 1 at 0:33
  • @TKinCA -- what do you want at this location? Remote control? Timer functionality? Something else? Feb 1 at 0:36
  • Timer functionality is the minimum. If it adjusted automatically to the seasons, great (but not required). If it worked with Kasa plugs, great (but not required). I did see a Intermatic ST01, which expliclity does not require a neutral and explicitly does work with CFL / LED / etc. That might be my best option.
    – TK in CA
    Feb 1 at 0:39

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