First Alert CO alarm: 3 chirps every minute is the malfunction signal
Three quiet chirps every minute is First Alert’s malfunction signal on its CO and combination alarms: the alarm is not working properly and has to be replaced (under warranty, First Alert replaces it). Do not confuse it with three loud beeps, a pause and three again, which on a combination alarm is the smoke alarm.
If it is sounding a full alarm: First Alert’s carbon monoxide instruction
Carbon monoxide alarm
First Alert (SCO5, SMCO210 and CO710 manuals): “If you hear the CO alarm horn and the CO red light is flashing, move everyone to a source of fresh air.” Then: press the Test/Silence button; “call your emergency services, fire department or 911”; “immediately move to fresh air—outdoors or by an open door or window” and do a head count. Do not go back in until the emergency services responder has arrived, the premises have been aired out and the alarm is back to normal. If it sounds again within 24 hours, repeat those steps and call a qualified appliance technician.
From Model SCO5 manual, Model SMCO210 manual, Model CO710 manual.
Fault, model by model
4 First Alert models whose manuals we read. The model number is on the label on the back of the alarm. If yours is not here, its own manual is the authority: First Alert models do not all use the same lights.
| Model | What you see | What you hear |
|---|---|---|
| CO400 battery CO alarm, one 9-volt battery | Red light flashing | 3 chirps every minute |
| SCO5 combination smoke and CO alarm, two AA batteries | Three rapid flashes with the chirps (the manual names no colour) | 3 chirps every minute |
| SMCO210 combination smoke and CO alarm, 10-year sealed battery | Three green flashes about once a minute | 3 chirps every minute |
| CO710 CO alarm with digital display, 10-year sealed battery | Display shows “Err” | 3 chirps every minute |
First Alert CO400: battery CO alarm, one 9-volt battery
- What you see
- Red light flashing
- What you hear
- 3 chirps every minute
“The light flashes (RED) and the horn sounds 3 ‘chirps’ every minute. MALFUNCTION SIGNAL. CO Alarm needs to be replaced.”
What the CO400 manual says to do
- Replace the CO alarm. “CO Alarms under warranty should be returned to manufacturer for replacement.”
Source:
- First Alert (BRK Brands): Battery Operated Carbon Monoxide Alarm with Silence Feature — User’s Manual — Model CO400 (manual M08-0140-029). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
First Alert SCO5: combination smoke and CO alarm, two AA batteries
- What you see
- Three rapid flashes with the chirps (the manual names no colour)
- What you hear
- 3 chirps every minute
“Horn does three ‘chirps’ every minute; LED has 3 rapid flashes with ‘chirps’. MALFUNCTION SIGNAL. Device is not working properly, and needs to be replaced.”
What the SCO5 manual says to do
- Replace the alarm; units under warranty go back to the manufacturer for replacement.
- The loud “3 beeps, pause, 3 beeps, pause” with the Power/Smoke light flashing red is this alarm’s smoke alarm, not a fault: get out.
Source:
- First Alert (BRK Brands): Combination Carbon Monoxide & Smoke Alarm — User’s Manual — Model SCO5 (manual M08-152696-SCO5-US). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
First Alert SMCO210: combination smoke and CO alarm, 10-year sealed battery
- What you see
- Three green flashes about once a minute
- What you hear
- 3 chirps every minute
“If Alarm is Not Operating Properly (MALFUNCTION SIGNAL) — Horn: 3 chirps every minute; Power LED: Three Green flashes approximately once a minute.”
What the SMCO210 manual says to do
- Replace the alarm. “If unit is under warranty, contact Consumer Support to process a warranty replacement.”
- Deactivate the old one (break the tab, slide the switch to deactivate) before you dispose of it.
Source:
- First Alert (Resideo): Smoke & Carbon Monoxide Alarm, 10-Year Sealed Battery — User’s Manual — Model SMCO210 (manual M08-0600-000). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
First Alert CO710: CO alarm with digital display, 10-year sealed battery
- What you see
- Display shows “Err”
- What you hear
- 3 chirps every minute
“Co Alarm Requires Service (Malfunction Signal): The horn sounds 3 ‘chirps’ every minute. CO alarm needs to be replaced. ‘Err’ is displayed.”
What the CO710 manual says to do
- Replace the CO alarm: “Based on its Self Test diagnostics, the unit has detected a malfunction. The unit needs to be replaced.” Under warranty, return it to the manufacturer.
Source:
- First Alert (BRK Brands): Carbon Monoxide Alarm, 10-Year Sealed Battery, with Digital Temperature Display — User’s Manual — Model CO710 (manual M08-0534-160326-US Rev B). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
Not the signal you have?
Sources
Every line on this page comes from First Alert’s own user’s manuals, read on the date shown; the quotes are the manuals’ words. firstalert.com’s product pages link these manuals as their downloads; the files are kept on the asset host of Resideo, which owns First Alert.
- First Alert (BRK Brands): Battery Operated Carbon Monoxide Alarm with Silence Feature — User’s Manual — Model CO400 (manual M08-0140-029). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
- First Alert (BRK Brands): Combination Carbon Monoxide & Smoke Alarm — User’s Manual — Model SCO5 (manual M08-152696-SCO5-US). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
- First Alert (Resideo): Smoke & Carbon Monoxide Alarm, 10-Year Sealed Battery — User’s Manual — Model SMCO210 (manual M08-0600-000). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
- First Alert (BRK Brands): Carbon Monoxide Alarm, 10-Year Sealed Battery, with Digital Temperature Display — User’s Manual — Model CO710 (manual M08-0534-160326-US Rev B). digitalassets.resideo.com Checked 2026-10-13. Linked as the manual from First Alert’s product page www.firstalert.com Checked 2026-10-13.
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