I inherited an existing installation of four smoke/CO detectors. They are all First Alert 3-wire AC-powered units. The test button on each detector produces some weird behaviors:

  • Some detectors only sound locally, without producing alarms elsewhere.
  • One detector sounds locally and also produces alarms all over the rest of the house (very loud; I can't tell for sure if every detector is sounding).
  • One detector doesn't sound locally, but produces alarms elsewhere in the house.

Something is obviously amiss here, and I intend to figure it out. Before I accuse the detectors themselves of being faulty and replace some/all of them (they are only two years old), I want to verify the integrity of the interconnect wire that they all share. Problem is, I can't do a simple continuity check because I don't have multimeter probes (or arms) nearly long enough to stretch across the house.

I'm thinking, after cutting the breaker, of temporarily tying the interconnect wire at one of the detectors to either hot, neutral, or ground and verifying that same continuity exists within all the other detector boxes, but I don't know if that's the best way to go about it.

Anybody have any better ideas for determining if my problem is the detector(s) or the wiring?

  • Have you read the First-alert manual and determined how they're supposed to work? Devices like these, which do not have displays or WiFi interfaces, typically communicate diagnostic information by intentionally unusual behavior. You must have the manual to decode this. Aug 18, 2017 at 20:49
  • 1
    @Harper Yup, manual briefly talks about differences in LED patterns to find the initiating/latching alarm, as well as CO conditions not sounding on non-CO alarms. Nothing about this, though.
    – smitelli
    Aug 18, 2017 at 21:58
  • Are they all the same model? Some detectors aren't compatible with each other, even if they're the same brand. If you have different models, consult the instructions and/or the manufacturer to verify compatibility.
    – mrog
    Oct 8, 2018 at 18:08

1 Answer 1


First I would check the wiring to the smoke detectors. You should have three wires, one white (neutral), black (hot) and another color usually red (the interconnect between the smoke detectors). Then I would verify that they are all pigtailed and are going to each individual smoke detectors. While I had them down I would also check the batteries and make sure they are charged. May just replace them to be sure. If everything checks out you should be able to get all of them to go of by pushing the test buttons. First the one you pushed the test button on then after a slight delay the others should start alarming.

If you test them and each one is sounding locally, then the interconnect is not connected between the detectors.

After you test them and one is not alarming, I would say it's probably bad and needs replacing.

  • Many units also now have indicator lights that show which unit was activated. Your instruction manual may offer more insight for proper testing of the system. Mine all have yellow wires for the interconnect, not that it matters (e.g., red, yellow, blue).
    – Upnorth
    Aug 18, 2017 at 17:27

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