Your Carbon Monoxide Detector Is Going Off: Exactly What to Do

If your carbon monoxide alarm is making a continuous four-beep pattern right now, stop reading: get everyone — including pets — out of the house immediately, go to fresh air, and call 911 from outside. Do not go back inside until fire department personnel tell you it is safe.

Carbon monoxide (CO) is an odorless, colorless gas. You cannot see it, smell it, or taste it. When your CO detector sounds, it has detected a gas that can kill you without warning. Treat every alarm as a real emergency first and investigate second.

SAFETY ALERT

If your CO alarm is beeping in a repeating four-beep pattern, leave the home right now. Call 911 from outside. Do not try to locate the source first.

What to do right now

Follow these steps in order.

Do not ignore it or assume it is a false alarm. The CPSC is clear: if your CO alarm activates, treat it as an emergency. Every second you spend second-guessing it is a second you are breathing potentially dangerous air.
Get everyone out — including pets. Wake up anyone sleeping. CO is especially dangerous for people who are asleep because they may not feel symptoms in time to react.
Get to fresh air immediately. Move away from the house. A neighbor's yard, the sidewalk, your car parked away from the garage — anywhere outside the building. Do not stop in the garage or a covered porch.
Call 911 or the fire department from outside. Firefighters have meters that measure CO levels and can find the source. Let trained responders handle the search.
Do not go back inside until you are cleared. Not even for a moment, not even for your phone or wallet. CO levels inside may still be high, and you can lose consciousness quickly with little warning.
Seek medical care if anyone has symptoms. Headache, dizziness, nausea, confusion, or trouble waking up — go to an emergency room. Tell staff you may have been exposed to CO. Oxygen treatment works, but only if you get there.

4 beeps vs. 1 chirp: what the sounds mean

Not every noise from your CO detector means an emergency. Knowing the difference matters.

Four beeps repeating: CO emergency. This is the standard alarm pattern used by major manufacturers including Kidde and First Alert. Four short beeps, a pause, four more — that means the sensor has detected dangerous carbon monoxide. Leave immediately.

One chirp every 30 to 60 seconds: not an emergency, but act on it. A single periodic chirp means the unit has a low battery or has reached the end of its useful life. You do not need to evacuate, but you do need to replace the battery or the whole unit. A dead detector cannot protect you.

End-of-life warning: Most CO alarms last five to seven years. Kidde units begin chirping every 30 seconds around the seven-year mark. Check the manufacture date on the back of your detector and replace it on schedule.

When in doubt about which pattern you are hearing, treat it as a CO alert until you know otherwise.

Symptoms of CO poisoning

CO poisoning is often called the "silent killer" because early symptoms feel like the flu — except there is no fever.

According to the CDC, the most common symptoms are:

Headache — often the first sign
Dizziness or lightheadedness
Nausea or vomiting
Weakness or fatigue
Confusion
Chest pain in more serious cases

A key clue: if several people in the household feel sick at the same time, or if symptoms improve when you leave home and come back when you return, CO is a strong possibility. The flu does not behave that way.

Symptoms worsen the longer you are exposed. High levels can cause loss of consciousness with little warning. Do not wait to see if you feel better — go outside and get medical help.

What causes CO in a home

Carbon monoxide is produced any time a fuel — natural gas, propane, oil, wood, gasoline — burns incompletely. The most common sources in a home are:

Furnace or heat exchanger problems. A cracked heat exchanger in a gas or oil furnace is one of the most dangerous CO sources in a home — combustion gases can leak directly into your ductwork. See our guide to a furnace burning or gas smell and other warning signs.

Gas water heater. A water heater that is not venting properly can release CO into living areas.

Blocked or damaged flue or chimney. Animal nests, debris, or corrosion can block vents that are supposed to carry combustion gases outside. When gases cannot escape, they back up into the house.

Car or other engine running in an attached garage. Even with the garage door open, an idling engine can push dangerous levels of CO through walls and gaps in minutes.

Portable generators used indoors or in garages. Generators produce large amounts of CO very quickly. The CPSC warns they should never be used inside a home, garage, or crawl space. Generator-related CO poisoning kills dozens of people every year, especially after storms.

Gas stoves, ovens, and grills. A gas appliance in a poorly ventilated space, or a grill brought inside, can raise CO to dangerous levels quickly.

After it is safe: get the source found

Once the fire department clears your home, do not reset your alarm and go back to normal. Your detector alarmed for a reason.

Before using any gas appliance again, have a qualified technician inspect all fuel-burning equipment and find the source. For a furnace issue, that means a licensed HVAC tech. For a water heater or gas line, your gas utility may have an emergency inspection line. Do not run any appliance until a pro confirms the system is safe. If the alarm goes off again within 24 hours after you return, leave and repeat all the steps above.

How to prevent CO problems

The best CO emergency is one that never happens. These steps significantly reduce your risk.

Install CO alarms on every level of your home and outside each sleeping area. The CPSC recommends placing alarms outside every separate sleeping area so they can wake you up at night. One detector on the main floor is not enough.

Replace alarms on schedule. Most CO detectors need to be replaced every five to seven years. Check the manufacture date on the back, write the replacement year on it with a marker, and stick to it.

Test your alarms monthly. Press the test button on each unit to confirm the alarm sounds.

Schedule an annual furnace and appliance inspection. The CPSC, NFPA, and most gas utilities all recommend a qualified technician inspect your furnace, water heater, flue, and vents before the heating season. A cracked heat exchanger or blocked vent may show no other sign until CO builds up.

Never run engines or grills indoors. Cars, lawnmowers, snowblowers, generators, charcoal or gas grills — all of these produce CO and should never run inside a home, garage, or enclosed space.

Keep vents and flues clear. Check exterior vents each season for bird nests, debris, or ice buildup. When in doubt, have a professional inspect them.

FAQ

My CO alarm went off but I feel fine. Do I still need to leave?

Yes. CO can reach harmful levels before symptoms appear, especially if you have been sleeping or the buildup was slow. The CPSC is explicit: if the alarm sounds, get outside. Do not wait to see if you feel sick.

Can cold weather or humidity trigger a false CO alarm?

True false alarms from environmental factors are rare in modern detectors. If your alarm sounds, assume it is real CO, leave, and let the fire department verify.

My alarm chirped once and stopped. Should I be worried?

A single chirp followed by silence is most likely a low-battery warning. Replace the battery and monitor the unit. If it happens again or changes to a repeating pattern, take it seriously.

How soon can I return home after a CO alarm?

Only return when emergency personnel confirm CO levels are safe and the source has been identified. Do not go back in just because the alarm stopped — the battery may have died or levels may have dropped temporarily.

Get the source inspected by a licensed HVAC pro

A CO alarm that went off means something in your home needs a professional set of eyes. A licensed HVAC technician can inspect your furnace, heat exchanger, and venting to find the problem before it happens again.

We connect homeowners with licensed, vetted HVAC contractors. Get a free quote and have the right person check your system before you turn the heat back on.

Sources

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If you smell gas or suspect a carbon monoxide leak, leave the area immediately and call 911 or your gas utility from a safe location.