After replacing the wired fire alarms in my house with new ones, when testing any of them, that one beeps but not the others.

Model is BRK First Alert 9120B6CP.

Each alarm came with three cables. Black and white for power, and orange for sending the signal that should cause all of them to go off when one of them does.

I have verified they are all properly wired and that they all have power (green light is on for all).

Any ideas on what else to check or verify to have them all communicate?

  • Manufacturer/model #? Oct 8, 2018 at 16:50
  • 1
    I just added to the post, @manassehkatz. Thanks for pointing out it was missing.
    – ceiroa
    Oct 8, 2018 at 17:05
  • Are all of the alarms the same model? Mixing models can cause problems like this.
    – mrog
    Oct 8, 2018 at 18:01
  • Did the previous alarms communicate with each other successfully?
    – mrog
    Oct 8, 2018 at 18:01
  • IIRC, Last time I tested them (about a year ago) they communicated with each other. When I tested them again they were not doing it anymore. Of the previous ones I had, one was older. Changed that one and they still weren't working. So I went ahead and changed them all (even most were only 4 years old) to the same newer model. Still nothing. I'm stumped.
    – ceiroa
    Oct 8, 2018 at 18:39

1 Answer 1


It turned out to be a short-circuit. The cable connecting one of the alarms in the basement had been trapped by the electrical box cover. The cover is metallic, and had cut into the plastic enough to make contact.

To find the issue I turned off the breaker, disconnected the signal cable from all of them, and started testing them from the top of the house using a 9V battery, as mentioned in one of the comments (+ to orange, - to white). Started with two, verified it was working, and kept going connecting alarms to be signal cable until I found out the culprit. After detailed inspection of the one that was causing the issue I noticed the cable damage.

Thank you @ThreePhaseEel for the 9V battery trick. It was really useful.

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