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1) The yellow wire is used as the "inter-connected" wire. This yellow wire should be connected to other smokes that also have an inter-connected wire. The black wire is the hot wire, the white wire is the neutral.
2) There are a few reasons as to why a smoke detector will go into a chirping mode. One common reason is a loose white neutral wire connection or faulty splice.
Check out the following link from FireX and Kidde as to the causes of the chirping:
3) If your smokes are 10 years old or older, they also need to be replaced and have reached their end-of-life. All smoke manufacturers recommend replacing the smokes at a maximum of 10 years.
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