Carbon Monoxide Alarm Going Off: What to Do Right Now, and What Comes Next

Carbon monoxide is a colorless, odorless gas made whenever fuel burns incompletely. At high levels it can kill within minutes, and an alarm is the only safe way to detect it. The CDC counts more than 400 unintentional, non-fire CO deaths in the U.S. every year.

Below: telling the alarm from a chirp, where CO comes from, what to do after the all-clear, and when the alarm itself is the problem. If you smell gas, that is a different emergency with the same first move: get out and call from outside.

Alarm or Chirp? Know the Difference

The CO alarm pattern is four quick beeps, a pause, and repeat. Kidde describes four short beeps and five seconds of silence; First Alert says four beeps and a pause means move to fresh air and call 911. Patterns can vary by model, so read your alarm's label or manual now.

On a combination smoke and CO alarm, smoke is three long beeps and a pause. The Kidde P3010CU also says "Fire!" for smoke and "Warning! Carbon monoxide!" for CO, and First Alert's ZCOMBO flashes a separate red light for each hazard. Either pattern means everyone leaves.

Chirps are maintenance signals, not alarms:

• Low battery: usually one chirp about once a minute, and "Lb" on Kidde's digital displays. On a Kidde CO alarm, a chirp every 30 seconds means end of life or a malfunction, not a low battery, and if fresh batteries don't stop any chirp, treat it as end of life. A sealed 10-year alarm like the P3010CU has no battery to change, so its low-battery chirp means replace the alarm.
• End of life: the sensor has worn out, and Kidde warns that an alarm in end-of-life mode can no longer detect CO. Kidde CO alarms chirp every 30 seconds (digital ones may show "ERR" or "END"), the P3010CU chirps twice every 30 seconds, and First Alert CO alarms chirp five times a minute. A new battery won't stop it.
• Malfunction: three chirps a minute on First Alert's ZCOMBO; a chirp every 30 seconds with a flashing fault code on the P3010CU. Replace the alarm or call the maker.

One rule for digital displays: Kidde says that any time a digital CO alarm shows a number, with or without the alarm sounding, assume it senses carbon monoxide, leave, and call 911. Our chirping alarm guide has the chirp fixes, and how to test smoke and CO alarms covers the monthly test.

Most Likely Sources of CO in a House

Only something that burns fuel makes CO. These are the usual sources on the CPSC, NFPA, and manufacturer lists; finding yours is a job for responders and a technician.

1. A furnace or boiler. A cracked heat exchanger, or a blocked, corroded, or disconnected flue or chimney, can send combustion gases into the house. That is why the CPSC wants heating systems, chimneys, and vents inspected and serviced every year.
2. A gas water heater. A corroded or disconnected vent pipe, or exhaust backdrafting down the vent, can spill CO into the basement or utility room.
3. A gas range or oven. Never use one to heat the house, per the CPSC and NFPA. The CPSC also warns that foil lining the bottom of a gas oven blocks combustion air and can produce CO.
4. A fireplace or wood stove. A closed damper, or a chimney blocked by debris or nests. The CPSC says to open the damper before lighting a fire and keep it open until the ashes are cool.
5. A car running in an attached garage. The NFPA says to pull the car out right after starting it and never run a vehicle or other engine indoors, even with the garage door open.
6. A portable generator too close to the house. The CPSC says outdoors only, at least 20 feet from the house with the exhaust pointed away, and never inside the home or garage, even with doors and windows open; generators lead its CO death figures. Our portable generator guide covers models with automatic CO shutoff.
7. Vents buried by snow. The NFPA and the U.S. Fire Administration say to keep the furnace, dryer, stove, and fireplace vents clear during and after a storm, and the NFPA adds that a running car's tailpipe must not be buried either. On a high-efficiency furnace, check the plastic intake and exhaust pipes too.
8. A fuel-burning space heater. Portable heaters ranked second to generators in the CPSC's 2020 CO death figures, and the CPSC says never to run an unvented fuel-burning appliance where people sleep. Our space heater guide goes further and recommends only electric heaters indoors.

After the All-Clear

Responders can test the air and look for the source; Kidde recommends calling 911 or the fire department for exactly that. Once they let you back in:

• Don't restart the likely source until it has been fixed. The CPSC says a malfunctioning appliance stays off until trained personnel service it.
• Have a qualified technician inspect the fuel-burning appliances before you use them again, vents and chimney included. Tell the technician what the alarm did and, on a digital model, its peak reading.
• Check the garage. The CPSC and both manufacturers say to confirm that no vehicle is running, or has been running, in an attached garage or next to the house.
• If the alarm sounds again within 24 hours, repeat the emergency steps and call a qualified appliance technician, per the CPSC and the Kidde and First Alert manuals.
• Don't just reset it and move on. Resetting quiets the horn; it doesn't fix whatever made the CO.

Anyone who had symptoms, even mild ones, should see a doctor promptly and mention CO, per the CPSC and First Alert.

False Alarm? Be Careful with That Word

Treat every CO alarm as real; the CPSC, Kidde, and First Alert all say never to ignore one. Alarms listed to the UL 2034 standard are built to stay quiet at low everyday levels (a steady 30 parts per million for 30 days) and to sound when CO climbs and stays up: within 60 to 240 minutes at 70 ppm, or 4 to 15 minutes at 400 ppm. Feeling fine proves nothing: First Alert notes that a healthy adult may feel no symptoms when the alarm sounds, and the CDC warns that sleeping people can die before they have symptoms.

"Nothing found" isn't proof of a false alarm, either. First Alert notes that CO can clear before an investigator arrives, and that some CO problems come and go with wind, exhaust fans, or loose vent pipes. If alarms keep happening with nothing found, First Alert says to check the alarm's location and have the house rechecked for an intermittent CO problem.

Two things about the alarm itself do cause trouble:

• Age. CO sensors wear out. Kidde gives its CO alarms a 7- to 10-year life depending on the model; First Alert says many last 5 to 7 years, and sealed 10-year models a decade. Go by the replace-by date on your unit.
• Placement. The NFPA says a central location outside each sleeping area and on every level, following the maker's instructions. The CPSC advises against kitchens, spots above fuel-burning appliances or near heating vents, and anywhere furniture or drapes can cover the alarm; Kidde says at least 15 feet from fuel-burning appliances.

When to Replace the Alarm

Replace a CO alarm at the replace-by date on its label, at an end-of-life or malfunction signal, when it still fails its button test after fresh batteries, and when a sealed-battery unit chirps low. The CPSC notes that the test button checks the circuitry, not the sensor's accuracy, so the date matters even when the test passes. Replace a hardwired alarm like for like; our smoke and CO alarm guide covers which type goes where.

The CDC suggests considering a detector with a digital readout, which can show the highest CO level in your home as well as sounding the alarm.

Prevent It

• Service fuel-burning appliances every year. The CDC and CPSC say to have the heating system, water heater, and other gas, oil, or coal appliances serviced by a qualified technician annually, chimneys and vents included. Book the furnace tune-up before heating season; the winter maintenance guide schedules it.
• Clear vents after every snowfall.
• Never run an engine indoors. No car, generator, mower, or pressure washer running in the house or garage, even with the door open.
• Never heat the house with the oven or range, and never use a charcoal grill or camp stove indoors or in the garage.
• Put a CO alarm on every level and outside each sleeping area, battery-operated or with battery backup, interconnected if you can, and test it monthly.
• Know the number. The NFPA suggests asking your fire department's non-emergency line ahead of time what number to call if the CO alarm sounds.

Frequently Asked Questions

Should I open the windows when the CO alarm goes off?

Get out first. The CPSC says to move outside to fresh air and not to go back in until responders give permission. The NFPA's wording allows "a fresh air location outdoors or by an open window or door," and Kidde says that only if you can't leave should you open doors and windows, turn off possible CO sources, and wait by an open door or window. Don't stay inside, or go back in, to air the house out.

Can I press the button to stop the alarm?

Kidde and First Alert manuals list pressing the Test/Hush or Test/Silence button as a first step, and it quiets the horn, but it does nothing about the CO. Kidde's P3010CU manual says the alarm reactivates if CO is still present, and that a repeat alarm within six minutes means high levels. Silencing never replaces leaving, and First Alert says not to remove the batteries.

What CO level is dangerous?

It depends on concentration, time, and the person. The CPSC says symptoms become noticeable above 70 ppm, and sustained levels above 150 to 200 ppm can cause disorientation, unconsciousness, and death. The CDC says infants, older adults, and people with heart disease, anemia, or breathing problems are more likely to get sick.

The Bottom Line

Four quick beeps and a pause means get everyone out, pets included, call 911 from outside, and stay out until responders say it is safe. Get medical care for anyone with symptoms. Have a qualified technician inspect the fuel-burning appliances before you use them again. A chirp is a battery or end-of-life signal, not an alarm, and an expired alarm can't protect you, so replace it by its date.

Sources