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First Alert

FIRST ALERT SC9120B Corded Electric Combination Alarm with Adapter Plugs for Easy Replacement

  • Based on 366 reviews
Condition: New
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Availability: In Stock.
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Arrives Apr 30 – May 4
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Style: Hardwired


Pattern Name: Alarm


Features

  • Convenient adapter plugs makes it easy to replace existing hardwired alarms without needing to rewire
  • Keep your family safe with this hardwired First Alert smoke and carbon monoxide alarm with a 10-year battery backup, which means constant monitoring for the life of the alarm, even if there's a power failure
  • Combination smoke alarm and carbon monoxide detector features electrochemical carbon monoxide sensing technology, as well as intelligent smoke sensing technology to reduce nuisance alarms
  • Indicator lights on the face of the unit display the presence of smoke or carbon monoxide, while an 85-decibel siren provides a clear, loud warning upon detection
  • Simple to use, with a single test/silence button; includes 10-year lithium battery with a locked drawer to prevent tampering
  • 10-year limited warranty; equipped with end-of-life signal, so you know when it's time to replace the unit for your safety
  • Rigorously tested to meet UL standards
  • To ensure complete protection make sure to place one carbon monoxide detector on each level of your home and in each bedroom

Description

This First Alert hardwired smoke alarm and carbon monoxide detector features an ionization smoke sensing chamber and an electrochemical carbon monoxide sensor. Installation and connection to AC power is easy with the included adapter plugs and "quick-connect" wiring harness so you can you easily replace existing hardwired alarms without needing to rewire. The 10-year battery backup allows for continued monitoring for the life of the alarm. Intelligent sensing technology is designed to reduce nuisance alarms; one button silences them if they do occur, and also tests detector functions. The latching alarm indicator automatically identifies which unit initiated the alarm, even after the conditions that set it off have subsided. And an end-of-life signal enhances your safety by letting you know when it's time to replace the unit. Connect it to compatible BRK or First Alert alarms within a single-family residence; connecting will sound all the alarms together.

Brand: First Alert


Style: Hardwired


Power Source: Corded Electric


Color: white


Product Dimensions: 0.07"D x 0.22"W x 0.22"H


Item Weight: 0.63 Pounds


Alarm: Audible


Operating Humidity: 10-95% RH


Sensor Type: Electrochemical


Global Trade Identification Number: 86


Power Source: Corded Electric


Alarm: Audible


Sensor Type: Electrochemical


Operating Humidity: 10-95% RH


Product Style: Hardwired


Color: white


Item Dimensions D x W x H: 0.07"D x 0.22"W x 0.22"H


Item Weight: 0.63 Pounds


Enclosure Material: Plastic


Brand: First Alert


Model Number: SC9120B


Global Trade Identification Number: 86


Manufacturer: First Alert


UPC: 029054018786


Item Type Name: Combination Alarm


Built-In Media: Alarm, Battery


Manufacturer Warranty Description: Warranty


Frequently asked questions

If you place your order now, the estimated arrival date for this product is: Apr 30 – May 4

Yes, absolutely! You may return this product for a full refund within 30 days of receiving it.

To initiate a return, please visit our Returns Center.

View our full returns policy here.

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Top Amazon Reviews

    • A great replacement for an old First Alert detector
Style: Battery Pattern Name: Alarm
I was looking for a replacement smoke/CO detector since my old detector had to be replaced after 8 years (which I thought should be 10 years). The old detector started chirping because of low battery. However when I replaced the battery, it was still chirping, but this time it was 5 beeps every minute. Looking at the back of the detector I saw the manufacturing date of 2014 but I also noticed the warning that 5 beeps means it needs to be replaced. The old detector is a First Alert combo with a 9v battery backup. The battery itself needs to be replaced every 6 months, which is really a hassle as well as it costs $8 for a pack of 2. Thus I wanted to find a new combo detector with a 10-year lithium battery. This is probably the best option since the battery lasts as long as the detector itself. The whole unit can be replaced every 10 years. This also saves a lot on the battery replacement itself. Plus the unit is just $10 more than the one with the 6-month battery backup. Installation was very easy since this unit has the same size as the old one, so I just use the old bracket since it fits perfectly. Also the old wire connector plugged in perfectly into the slot in the new unit. This is why I chose the same brand, to ensure perfect replacement fit. The only thing I had trouble with was the tab, which needs to be pulled out to activate the battery. It was very hard to pull it out, so I had to open the battery compartment (which was pin-locked) and pull out the tab completely. So my advise is, if you plan to replace your combo detector, just choose the same brand to ensure compatibility and easy installation. I would recommend this unit since it has a 10-year battery backup for peace of mind. Hopefully this will last 10 years as promised. That's a battery saving of $40 right there for a new unit like this in another 10 years. I also plan to get another 3 units of this model to replace when I hear another low battery beeps of the old ones. ... show more
Reviewed in the United States on March 22, 2022 by Localshopper
    • Easy to install
Style: Hardwired Pattern Name: Alarm
Works great
Reviewed in the United States on March 16, 2026 by Don Grubbs
    • Replacement
Style: Hardwired Pattern Name: Alarm
Perfect replacement for original detector
Reviewed in the United States on February 2, 2026 by legj4
    • One of mine failed in 4 years, but other then that they work very well.
Style: Battery Pattern Name: Alarm
Everything is needed in kit, I would like manufacture to use solid pair copper so you can twist together with home wiring as the stranded can pop free from wire-nut if not careful. That said my sons room started beeping for replacement battery at only 4 years. I went ahead and ordered a full set to replace that one. All this said most smoke alarms require you to replace a 9V every 6 months or so, when factoring that these are still much much more convenient then normal smoke alarms and the replacement integrates perfectly as I was able to do test and all alarms responded. I am holding off on doing the others at moment to see if the one was just a dud but will likely do them within a month so they are all the same age. A couple are very high and difficult to get to which is why I've just done the defective one at moment. In any case I am mostly satisfied other then not getting 9-10 years out of internal lithium battery but I am sure where they are sourced matters and I got a dud. The alarms themselves work flawless and connect to both home wiring and have a battery backup so I consider these robust and a great safety measure to protect your family. So note to manufacturer make sure premium lithium batteries are used but other then that these work great. ... show more
Reviewed in the United States on December 22, 2025 by J. Harmon
    • Easy to install.
Style: Hardwired Pattern Name: Alarm
Came in as specified. Was easy to install.
Reviewed in the United States on February 13, 2026 by Bea
    • First Alert
Style: Hardwired Pattern Name: Alarm
It was exactly what I needed. Thank you!
Reviewed in the United States on March 22, 2026 by Hilda Holbrook
    • BRK branded First Alert detectors cover the bases
Style: Battery Pattern Name: Alarm
When I moved into my house, there were three contractor grade alarms with one just being battery powered, and two interconnected. However, the two AC models did not have battery backup?!?!? The other seemed like an afterthought or they were forced to put one in to get it passed inspection. The house was built in 1991, but even then, that was inadequate. The first owner added 4 battery powered smoke alarms to the bedrooms. This menagerie of alarms and functionality was poor at best and key places in the house had no alarms. In 1998 when I moved in, I began rewiring the house due to very poor electrical layout. I decided to upgrade the alarms at the same time. I had a media circuit which I ran an alarm branch that began on the 2nd floor, then to the first floor and then to the basement floor. I wanted alarms that were AC, had a battery backup, and all of them interconnected together. BRK brand (at that time) was the only company that had smoke, co, combo smoke/co and heat alarms that would all work together. I installed the max that could be installed and interconnected on one zone: by code 12 smoke max, and 18 alarms max. I had most normal rooms with a smoke detector, hallways, stairs, and areas near gas combustion, I did combos, and in other areas where you can have smoke/co alarms (like the laundry room and garage), I did heat alarms. The battery backup on these was a 9-volt battery. It was annoying to replace these over the 10 year life span. It was time to update the alarms as some were failing and causing issues. First Alert has bought BRK. They still make the BRK alarms and even updated them with lithium batteries that last 10 years! This should eliminate the need for annoying chirps which my dogs hate. The wire harnesses are still the same so I just unclipped the old ones and clipped in the new ones - easy peasy! One annoying thing with the BRK alarms, they haven't updated the heat alarms like they did with the smoke and co alarms. However, unlike the smoke and co alarms (which continually sample the air), the heat alarms are analog per se and have a mechanical trip should there be a rapid rise in temperature. 9-volt batteries seemed to last longer in those. Also, as they are mechanical, they don't "wear out" like the electronics on the smoke and co alarms. This will be my second decade round with BRK alarms. ... show more
Reviewed in the United States on August 3, 2023 by Jeff in Seattle

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