Why is my carbon monoxide detector beeping?

A carbon monoxide detector that chirps once in a while usually has a low battery, has reached the end of its life, or has an error. A loud alarm that repeats, often 4 beeps and a pause, means it sensed CO: get everyone outside and call 911.

A loud, repeating CO alarm is an emergency, and so is any chirp when people feel sick, dizzy or headachy; get everyone and pets outside and call 911 from outside. A single chirp with everyone feeling fine is not an emergency, but fix it today, because an alarm in end-of-life mode can't detect CO (Kidde).

If your carbon monoxide alarm goes off, get everyone and pets outside into fresh air right away and call 911 or the fire department from outside. Don't look for the source first. Don't go back in until responders say it's safe (CPSC).

A carbon monoxide alarm usually sounds as 4 short beeps and a pause, over and over, while a smoke alarm sounds 3 beeps and a pause (NFPA's "learn the sounds" campaign). That's also how a smoke and CO combination alarm tells you which one it found, and many say "Fire" or "Carbon monoxide" out loud. Patterns can vary, so check your manual, and get everyone outside either way.

What 2, 3 or 5 beeps mean depends on your alarm's brand and model, so check the label or manual. If the beeps are loud and keep repeating, it's an alarm, not a chirp: on a combination alarm, 3 beeps and a pause usually means smoke and 4 means CO (NFPA), so get everyone outside and call 911. For chirps, First Alert, for example, says 5 beeps a minute means end of life.

A CO detector that still chirps after a new battery may have the battery in wrong, a door that isn't shut, or leftover charge that needs a reset. If none of those fix it, it's probably at the end of its life; Kidde says a new battery won't stop that chirp.

Treat an alarm that went off and stopped as real carbon monoxide: the CO may have drifted away when a door opened, or the source may have shut off for now. Get everyone outside and call 911 or the fire department. If it sounds again within 24 hours, have a qualified technician check every fuel-burning appliance (CPSC).

Most carbon monoxide detectors last 5 to 10 years, depending on the model: First Alert says 5 to 7, Kidde says 7 or 10, and the CDC says every 5 years or as the maker says. Go by the "Replace by" date on the label.

To stop a chirp, put in a new battery of the type on the label and press Test, or reset it: battery out, hold Test 15 seconds, battery back. An end-of-life chirp only stops when the alarm is powered off, so replace it (Kidde). Never silence a loud alarm and stay inside: get out and call 911.

Treat every carbon monoxide alarm as real, because you can't see or smell CO, so there's no way to rule it out from inside. Today's CO alarms are less likely to give false alarms than older models (CPSC). An alarm that goes off when the furnace, a space heater, the oven or the water heater turns on is a warning sign, not a fluke, since those are common sources of CO (CPSC). Get everyone outside, call 911 from outside, and have the appliance checked before you use it again.

A carbon monoxide detector does not detect a natural gas leak; it only senses carbon monoxide (Poison Control). Gas companies add a rotten-egg smell to natural gas so you can notice a leak. If you smell gas, don't light anything, get everyone outside, and call the gas company from outside.

If your carbon monoxide alarm is going off, call 911 or the fire department from outside, and the gas company too if you smell gas; afterward, an HVAC technician can check your furnace and gas appliances. Most chirping CO detectors don't need a pro: a new battery, a reset or a new alarm fixes them. Call an electrician for a hardwired alarm with no power or a plug that doesn't match. Renting? Tell your landlord.