Replacing a Kidde Fyrnetics 123/9HI Smoke Alarm

The alarm in question looks like this:

The red light indicates a fault, which is why I’ve finally confronted my inability to remove it (hopefully your smoke alarm fault-free).

I’ve found the Kidde instructions on this to be infuriatingly vague on how you actually detach the unit from the base, so as I had to go through a lot of swearing to work it out, I figured some other people might have the same problem. Why are instruction manuals often so badly written?

The side of the alarm has the arrows to twist clockwise for “off” and anti-clockwise for “on”, but there is a small lug which prevents the unit from twisting. Here’s the lug:

A Kidde Fyrnetics 123/9HI Smoke Alarm

Use a small screwdriver, or other poking device, to push this lug towards the base of the smoke alarm, like so, and you will be able to twist the unit free of the base:

Locking lug on Kidde Fyrnetics 123/9HI Smoke Alarm

The unit will still be attached to the base by the power plug. The plug has two catches on either site, like this, squeeze them together:

Catches on plug within a Kidde Fyrnetics 123/9HI Smoke Alarm

You can see the catches in this photo:

Plug within a Kidde Fyrnetics 123/9HI Smoke Alarm

The end. Remember to only remove your smoke alarms temporarily, and refit them as soon as possible!

(N.B: I believe this alarm is discontinued by Kidde.)

Join the Conversation


  1. Hi i have these now discontinued smoke alarms by kidde as shown above also, can i ask what you repalced them with and did you need a new base plate or not? Thanks..

    1. Similar to Andrea in contact with you on April 23, I am unsure where or how to get a replacement that
      will fit the 123/9 HI base, so would be grateful for your advice. I live in the Belfast area or might it be
      available from a web site?


  2. Simon, hi

    Did you replace them yet and if you have done, do you still have their old mounting plate?

    Needed to change the battery on one of mine and only found your ‘how to remove’ article here after I’d got mine off by in the end breaking the mounting plate, so if you’ve now got a couple of the plates you maybe don’t need… — what could I give you for them?



  3. Hi Simon,
    Thanks for these instructions. Managed to the battery out of the alarm without any problems. Speaking to Kidde Safe in the UK the unit is obsolete and you need a new bass plate for a new alarm.
    Thanks Again.

  4. Are they still for sale anywhere? Two of them stopped and can’t be repaired.
    Thanks for your help.
    Wolter Taekema, Holland

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