I have 4 smoke/fire and carbon monoxide detectors that are hard wired and have 9 volt battery back up system. They all went off in the middle of the night for no reason. The fire department came to my house, and told me to simply replace them. I went out and bought 4 new ones, installed them, and as soon as I turned on the circuit breaker the alarms started going off again. It's not dust, and the batteries don't need replacement, all 4 are brand new. What could be causing this problem?

Are you 100% positive you bought 4 that are designed to be connected so when goes off they all go off? These units are specially design. I do lots of this install and had some difficulty with install 4 in series. Which model/brand did you buy?

Kidde firex model#kn-cosm-ib

Page 13 goes over the proper wiring. Did you have this same model before and reuse the wiring "plug" or did you install a new plug. Here are a few other things. Remove all 120 vac plugs and test each unit with battery only, make sure all 4 work. Yesy between the black and the white wire, should get 120 vac.

You could power down that circuit, disconnect the interconnect wire at each(usually orange or yellow), and see which is detecting something.
They may be detecting something.
A Red Light May go on the detector that is triggering.
All else, see if you have voltage on all of the interconnect wires.
Any recent wiring?

Strat, this models interconnect is black, white, red. Pretty much like a 3 way traveler.

If Black, white and red, I have seen people reverse the power and interconnect wires.

Hard to do but not impossible. This model comes with a plug with white, black and red pigtail for you to tie in your lines. You'd have to make a pig with a black and a white. You did not do that did you Maggie?