First Alert Smoke/CO Detectors
First Alert Detectors model 9021B were installed in August 2018, during renovation. The manufacture date on the one from the hallway is June 26, 2018. On 7/25/2025 replaced them with model SC9120BA.
7/26/2025 – New unit in the Office began 3-chirps every minute. Called First-Alert and they are sending a replacement that will have a different model number as the ones we have are for Canada. He would not replace the other three. The units have a 10-year warranty.
Most CO alarms expire after 7 years. Our CO Alarms use electrochemical sensors precisely calibrated to determine PPM levels of CO in the air. These sensors degrade naturally over time and after about 7 years are no longer precise enough to be reliable in detecting exact PPM levels, leading to false alarms or not being able to detect elevated CO levels. From their web site here.
Refer to the chart to learn what “chirps” or “beeps” mean on CO alarms. Troubleshoot 1 chirp, 3 chirps, 5 chirps, or something different. From here.
| Chirp Pattern | This means… | You should… |
| The horn chirps 1x per minute. | Low battery warning. | Install a new Duracell, Energizer, or Everready battery. |
| The horn sounds 3 chirps every minute. | Malfunction warning. CO alarm needs to be replaced. | Alarms under warranty should be returned to manufacturer for replacement. |
| The horn sounds 5 chirps every minute. | End of life signal. CO alarm needs to be replaced. | Immediately replace the alarm. |
To determine which Smoke/CO Alarm initiated an alarm.
During an Alarm:
On Initiating Alarm(s) – Red LED(s) flashes (flash) rapidly
On All Other Alarms – Red LED is Off
After an Alarm (Latching):
On Initiating Alarm(s) – Green LED(s) On, Red LED(s) flash once every 5 seconds
On All Other Alarms – Green LED(s) On, Red LED(s) is O
How to Replace a 9V Battery
- Use the plastic tab to gently open the battery compartment.
- Remove the old battery.
- Look at your 9V battery to identify the positive (+) and negative (-) contacts similar to what is shown in the image. When installing the battery in your alarm make sure they match the same polarity in the battery drawer. Note: the negative side has a larger opening in the back of the battery drawer so it will only fit one way without forcing it closed.
- Insert the new battery into the compartment with the positive (circular ? ) contact closest to the center of the detector. The negative (hexagonal ? ) contact should be closer to the outside of the detector.
- Gently close the battery compartment. If it won’t close, the battery may be facing the wrong direction.
The image below is from the O&M info that is in DropBox/O&M Manuals that can be accessed via this link.
ChatGPT responses about a chirping detector on 9/16/2026
Prompt No. 1 – I replaced the battery in my First Alert Detectors model SC9120BA and it still emits a single beep and the green LED flashes. I replaced it with another new battery and it still does the beep/flash. What could be wrong?
With the First Alert SC9120BA, if you have installed a genuinely fresh 9-volt battery and it continues to give one chirp about once a minute accompanied by a green LED flash, there are a few possibilities.
The important point is that the green flash/chirp pattern does not necessarily mean the battery itself is bad.
What the SC9120BA is telling you
According to the SC9120BA manual:
| What you observe | Meaning |
|---|---|
| Green light continuously on, no chirp | Normal AC-powered operation |
| Green flashes about once/minute, no chirp | AC power is absent; running on battery |
| Green flashes about once/minute with one chirp | Low-battery warning |
| Green flashes 3 times with 3 chirps/minute | Malfunction — replace alarm |
| Green flashes 5 times with 5 chirps/minute | End of life — replace alarm |
So if you’re getting exactly one chirp + one green flash approximately every minute, the detector believes it has a low/missing battery.
Since you’ve already replaced the battery twice
I would check these things, in this order:
1. Make sure the battery is actually making good contact.
The SC9120BA uses a standard 9V battery. The manual specifically says to make sure the battery terminals match the terminals in the detector and that the battery compartment is properly closed. (manualzz.com)
Open the drawer and check for:
- A loose battery connector
- Corrosion on the contacts
- A bent contact
- The battery not being fully seated
- The drawer not completely closing
A new battery with a poor connection can produce exactly the symptom you’re seeing.
2. Check the AC power. This is particularly important with the SC9120BA.
The SC9120BA is a hard-wired AC alarm with a 9V battery backup. Under normal conditions, the green light should be continuously illuminated, not merely flashing. (ManualsLib)
If the green light is flashing once a minute, the manual says the alarm is not receiving AC power and is operating on its battery backup. (manualzz.com)
So if yours is flashing and chirping, I’d look at whether the 120-V AC supply to that particular detector has been lost.
For example:
- Circuit breaker tripped
- GFCI or other upstream protection tripped
- Loose wire connection
- The plug-in wiring connector on the back of the detector has become loose
- An electrical connection has deteriorated
Do not remove the detector or inspect household wiring with the circuit energized. First turn off the appropriate breaker.
3. The detector itself may be old
This is the other possibility I’d take seriously.
The SC9120BA has an end-of-life indication. However, end-of-life is normally five chirps and five green flashes per minute, not one. (ManualsLib)
A malfunction is normally three chirps and three green flashes per minute. (manualzz.com)
Therefore, one chirp/one flash points more strongly toward the battery/AC-power system than an electronic malfunction.
One very useful test
I’d do this before buying another detector:
- Turn the circuit breaker off.
- The green light should change because the alarm is now on battery backup.
- Turn the breaker back on.
- The green LED should become steadily illuminated under normal AC operation.
- Press and hold the Test/Silence button and verify that the alarm produces its normal test sequence.
The manual says that under normal AC operation the green LED should be continuously on; if AC isn’t reaching the alarm, it flashes once a minute. (ManualsLib)
My suspicion in your case
Because you’ve already tried two new batteries, I would not buy a third battery yet.
If the pattern really is:
one chirp ? one green flash ? approximately 60 seconds ? repeat
then I’d first investigate whether this particular SC9120BA is receiving AC power.
If you tell me whether the green light is normally steady or flashing, how often the chirp occurs (roughly every 60 seconds?), and whether you have other interconnected First Alert detectors that are behaving normally, I can narrow it down considerably.
Prompt: Without doing anything to it the green LED is now not flashing and it is not chirping. The other detectors also have a continous green LED. Can you explain what happened and is it all ok now?
Yes — if it has now stopped chirping and the green LED is continuously on, that is the normal condition. Your observation that the other detectors have a continuous green LED is a useful confirmation. The SC9120BA manual specifies continuous green for normal AC-powered operation. (ManualsLib)
What probably happened is this:
- The detector temporarily lost or failed to recognize its AC power, so it switched to battery backup.
- While operating on backup power, it detected the battery condition and began giving the warning chirp.
- You replaced the battery, but the detector may have needed some time to reset its battery-monitoring circuitry. [After replacing the battery and waiting 5+ minutes it reset it self.]
- At some point, AC power and/or the battery connection was recognized normally, and the detector returned to normal operation.
- Now that you have steady green + no chirp, it is behaving as it should.
There is an especially important distinction here: the SC9120BA’s documented low-battery condition is a chirp approximately once a minute with the green LED on for about 2 seconds and off for about 2 seconds. A loss of AC power produces a different indication. (ManualsLib)
What I would do now
Don’t change anything right now. Leave the new battery installed and watch it for the next day or two.
If it remains:
Green LED continuously ON + no chirping
then I would consider the detector to be operating normally.
However, if it starts doing the chirp/flash again, pay attention to whether the other detectors’ green LEDs remain continuously on. If only this detector starts flashing/chirping again, that would make an intermittent problem with this detector’s AC connection, battery contacts, or internal electronics more likely.
One additional thing I’d recommend: press the Test/Silence button once and let it complete its test. The SC9120BA should produce its normal test alarm sequence. If it tests normally and subsequently remains steady green and silent, that’s reassuring. (ManualsLib)
So, based on what you’re seeing right now: yes, it appears to be OK. I would not replace the detector solely because of what happened earlier.
