First Alert CO alarm: 4 beeps, a pause, 4 beeps
Four loud beeps, a pause, then four again, with a red light flashing, is the carbon monoxide alarm on First Alert’s CO and combination alarms: the sensor has found enough CO to trigger an alarm. It is not a battery or fault warning; those are quiet chirps once a minute. On a combination alarm, three beeps and a pause is smoke instead, and both need you out of the house first.
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.
Real alarm, 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 ALARM light flashing rapidly | Loud: 4 beeps, a pause, 4 beeps, repeating |
| SCO5 combination smoke and CO alarm, two AA batteries | CO light flashing red, Power/Smoke light off | Loud: 4 beeps, a pause, 4 beeps, repeating |
| SMCO210 combination smoke and CO alarm, 10-year sealed battery | Power light flashing red | Loud: 4 beeps, a pause, 4 beeps, repeating |
| CO710 CO alarm with digital display, 10-year sealed battery | Blue backlight on; display shows “CO”, the ppm number, a full level bar and “EVACUATE” | Loud: 4 beeps, a pause, repeating |
First Alert CO400: battery CO alarm, one 9-volt battery
- What you see
- Red ALARM light flashing rapidly
- What you hear
- Loud: 4 beeps, a pause, 4 beeps, repeating
“Sensor has detected enough CO to trigger an alarm. Light flashes rapidly and horn sounds loudly (repeating 4 beeps, pause).”
What the CO400 manual says to do
- Move everyone to fresh air and call 911 or the fire department (the instruction at the top of this page).
- The Test/Silence button quiets the horn for about 4 minutes: “After initial 4-minute Silence cycle, the CO Alarm re-evaluates present CO levels and responds accordingly. If CO levels remain potentially dangerous—or start rising higher—the horn will start sounding again.”
- “NEVER remove the batteries from your CO Alarm to silence the horn. Use the silence feature.”
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
- CO light flashing red, Power/Smoke light off
- What you hear
- Loud: 4 beeps, a pause, 4 beeps, repeating
“Carbon Monoxide (CO): CO LED: Flashes Red. Horn: 4 beeps, pause, 4 beeps, pause. Power/Smoke LED: Off.”
What the SCO5 manual says to do
- Move everyone to fresh air and call 911 or the fire department (the instruction at the top of this page).
- Silencing a CO alarm is short: “The CO alarm will remain silent for up to 4 minutes. After 4 minutes, if CO levels remain potentially dangerous the horn will start sounding again.” A smoke alarm on the same unit stays quiet for up to 15 minutes.
- The smoke pattern on this alarm is different: Power/Smoke LED flashing red, “3 beeps, pause, 3 beeps, pause”, CO LED off.
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
- Power light flashing red
- What you hear
- Loud: 4 beeps, a pause, 4 beeps, repeating
“Alarm Levels of CO are Detected — Horn: 4 beeps, pause, 4 beeps; Power LED: Flashes Red.”
What the SMCO210 manual says to do
- Move everyone to fresh air and call 911 or the fire department (the instruction at the top of this page).
- While silenced the horn is off and the Power LED keeps flashing red. “The CO Alarm will remain silent for up to 4 minutes. After 4 minutes, if CO levels remain potentially dangerous the horn will start sounding again.”
- If the CO alarm goes back into alarm 4 minutes after you silence it, First Alert says “CO levels indicate a potentially dangerous situation.”
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
- Blue backlight on; display shows “CO”, the ppm number, a full level bar and “EVACUATE”
- What you hear
- Loud: 4 beeps, a pause, repeating
“Blue backlight glows and horn sounds loudly (repeating 4 beeps, pause). … ‘CO’ alternating with the ppm number, a full level, and ‘EVACUATE’.”
What the CO710 manual says to do
- Move everyone to fresh air and call 911 or the fire department (the instruction at the top of this page).
- The peak level button shows the highest CO level recorded during an alarm; the manual says the reading is kept for 24 hours, which helps the responder or technician.
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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