The Firex appears to have two different connectors, both with the same pins. One connector seems to fit well and has a plastic locking mechanism that fits over these adapters, while the other Firext connectors prevent the retaining clip from slipping and holding the adapter in place. You won't know which one you have until you look at it - the ones that work have a small notch in the middle of the white plastic that allows the locking clip to slide in and out of the white Firex connector. The bad ones just won't connect to each other, making this adapter useless. Of course, First Alert support didn't say anything about it and only checks that this adapter works on all Firex 120-305B devices, which they don't. Read the reviews, some work, some don't.
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