A carbon monoxide (CO) alarm that sounds a repeating pattern of four loud beeps means CO has been detected: move everyone outside into fresh air right away and call emergency services. A single chirp every 30 to 60 seconds is usually a low-battery or end-of-life signal, not a CO alarm. When in doubt, treat it as real and get out first.
If it is an alarm: do this now
Move everyone, including pets, outside into fresh air immediately. Call emergency services (911 in the US) from outside. Do not go back in to find the source, and do not silence the alarm and carry on. Stay out until firefighters or the gas utility say it is safe. Anyone with symptoms needs medical attention even if they feel better in fresh air.
Alarm or chirp?
| What you hear | What it usually means |
|---|---|
| Four loud beeps, a pause, then four again | Carbon monoxide detected — evacuate |
| One short chirp every 30 to 60 seconds | Low battery |
| Chirps in a repeated pattern, or a chirp plus a fault light, on a unit several years old | End of life, or a sensor fault — replace the unit |
| Random beeping after a power return | Hardwired unit resetting; check the manual |
Patterns differ between brands, so check the manual or the label on the unit. If you cannot tell which signal you are hearing, treat it as a CO alarm.
Symptoms of CO poisoning
Early symptoms resemble flu without the fever: headache, dizziness, nausea, weakness, shortness of breath and confusion. Several people or pets in the home getting sick at the same time, or symptoms that improve when you leave the house, are warning signs. CO has no smell, color or taste, so a working detector is the only dependable warning.
Where CO comes from
- Fuel-burning furnaces and boilers with a blocked flue or cracked heat exchanger (see furnace smells)
- Gas water heaters and fireplaces with poor venting
- Gas stoves and ovens used for heating a room, which should never be done
- Generators, grills and camp stoves used inside or too close to a window or garage (see preparing for a power outage)
- A car left running in an attached garage
If it is just chirping
- Check the date on the back
Most CO alarms last roughly five to ten years; the replace-by date is printed on the unit. An expired unit should be replaced, not given new batteries.
- Replace the batteries
For battery or battery-backup models, fit fresh ones and press the test button.
- Replace the unit if chirping continues
A new battery that does not stop the chirp, or a unit past its date, means it needs replacing.
After the emergency
Do not re-enter until emergency responders say so. Have all fuel-burning appliances inspected by a qualified technician before you run them again, and do not use a suspect appliance until it is cleared. Report any symptoms to a doctor.
Placement and maintenance
Install a CO alarm on every level and outside sleeping areas, following the manufacturer’s guidance on height and distance from fuel-burning appliances. Avoid humid bathrooms and spots next to vents. Test monthly. Do not confuse smoke and CO alarms — see smoke detector false alarms and include them in your winterizing checklist.
When to call a professional
Call a qualified heating or gas technician after any real alarm, and an electrician for a hardwired unit that keeps misbehaving. Emergency services handle the alarm itself.
