If your carbon monoxide alarm is sounding a loud, repeating pattern of four beeps followed by a pause, it means the device has detected dangerous levels of carbon monoxide in your home. Get everyone outside immediately, then call 911 from a fresh air location. Do not go back inside until emergency responders clear the home.
Not every beep is an emergency, though. Carbon monoxide detectors make different sounds for different reasons, and knowing which pattern you’re hearing determines whether you need to evacuate or simply swap a battery.
What Each Beep Pattern Means
4 beeps followed by a pause, repeating: This is a true emergency. The sensor has detected carbon monoxide in the air. Leave the building right away.
1 beep every minute: Low battery. The detector is still functional but needs fresh batteries soon. Replace them promptly, because a dead detector can’t protect you.
5 beeps every minute: End of life. The sensor inside the unit has worn out and the entire detector needs to be replaced. CO alarms last about 7 years. Units made after August 2009 include this end-of-life warning automatically. If your detector was installed in 2018 or earlier, check the manufacture date printed on the back.
If you’re unsure which pattern you’re hearing, treat it as an emergency and get outside. You can sort out the details once everyone is safe.
Why Carbon Monoxide Is So Dangerous
Carbon monoxide is an odorless, colorless gas. You cannot smell it, see it, or taste it, which is exactly why detectors exist. When you breathe it in, CO enters your bloodstream through your lungs and latches onto hemoglobin (the molecule in red blood cells that carries oxygen) with roughly 200 times the grip that oxygen has. Once CO takes oxygen’s spot, your blood can no longer deliver oxygen to your brain, heart, and muscles. The result is a slow suffocation from the inside out, even while you’re still breathing normally.
Symptoms progress with exposure. At lower levels, you’ll notice a headache and nausea. As the gas builds up in your blood, you may feel dizzy, weak, and unable to concentrate or think clearly. Higher exposure causes confusion, chest pain, and difficulty breathing during any physical effort. At very high concentrations, loss of consciousness, seizures, and death can follow. In severe cases that don’t prove fatal, neurological problems like memory loss, personality changes, or movement disorders can appear days to weeks later and sometimes become permanent.
One reason CO poisoning is so insidious is that early symptoms (headache, fatigue, nausea) look a lot like the flu. If multiple people in your household develop similar symptoms at the same time, especially symptoms that improve when you leave the house and return when you come back, carbon monoxide should be high on your list of suspects.
What Triggers the Alarm
CO detectors don’t go off instantly at the first trace of the gas. They’re designed to mirror how the human body responds to exposure over time. Under the UL 2034 standard that most U.S. detectors follow, a unit must alarm within 60 to 240 minutes at 70 parts per million (ppm), within 10 to 50 minutes at 150 ppm, and within 4 to 15 minutes at 400 ppm. Lower concentrations that wouldn’t pose an immediate health risk are deliberately filtered out to avoid nuisance alarms.
So if your alarm has triggered, the CO concentration in your home has been high enough, for long enough, to represent a genuine health risk. This is not a drill situation.
Common Household Sources of CO
Any appliance that burns fuel can produce carbon monoxide. The most common culprits in homes include:
- Furnaces and boilers with cracked heat exchangers or blocked exhaust vents
- Gas water heaters with faulty venting
- Gas stoves and ovens, particularly when used for heating a room
- Fireplaces and wood stoves with blocked or damaged chimneys
- Unvented kerosene or gas space heaters
- Cars, generators, or lawn equipment running in or near an attached garage
A car idling in a garage can produce lethal CO levels even with the garage door open. Portable generators are another major source, responsible for a spike in CO deaths during power outages when people run them indoors or in enclosed spaces.
Most CO problems come down to poor ventilation or a malfunctioning appliance. A furnace that worked fine last winter can develop a cracked heat exchanger this year. A bird’s nest in a chimney flue can redirect exhaust gas back into the house. These aren’t things you’d notice on your own, which is why annual inspections of fuel-burning appliances matter.
What to Do When the Alarm Sounds
Move everyone outside or to fresh air immediately. Account for every person and pet. Do not waste time opening windows or trying to find the source. Call 911 from outside and tell the dispatcher your carbon monoxide alarm went off. Stay outside until the fire department arrives and checks the home with professional-grade CO monitors.
If anyone is experiencing symptoms like headache, dizziness, confusion, or nausea, tell the emergency responders right away. CO poisoning is treated with high-concentration oxygen, and faster treatment leads to better outcomes.
Fire departments typically clear a home for re-entry once their equipment shows CO levels below 30 ppm. If the source is identified (a malfunctioning furnace, for example), it will need to be shut down and repaired before you use it again. If the source isn’t immediately obvious, have a qualified technician inspect all fuel-burning appliances and your ventilation system before assuming the problem is resolved.
Keeping Your Detectors Reliable
A CO alarm that’s past its lifespan gives you a false sense of security. Replace every unit at the 7-year mark, regardless of whether it still appears to work. The electrochemical sensors inside degrade over time and eventually stop detecting CO altogether. Flip the detector over and check the manufacture date printed on the label.
Install detectors on every level of your home and near sleeping areas. Test them monthly using the test button. If your detector runs on batteries alone, replace them at least once a year. Combination smoke and CO units follow the same replacement schedule for the CO sensor, so don’t assume a working smoke alarm means the CO sensor is still good.
During heating season, pay extra attention. CO incidents spike in winter when furnaces run constantly, windows stay shut, and people bring supplemental heaters indoors. If your alarm goes off during a cold snap, there’s a strong chance your heating system is the source.