What Your Carbon Monoxide Alarm Is Telling You
Why this matters
A carbon monoxide alarm makes more than one sound, and the sounds mean very different things. One means get everyone out of the house now. Another means the battery is low. A third means the alarm itself has reached the end of its life and has stopped protecting you. People get these mixed up in both directions: they leave the house over a low battery at 3 a.m., which is a nuisance, or they pull the battery out of an alarm that was sounding for real, which can be fatal.
This card teaches you to read your alarm: the sound patterns, the label on the back, and what the alarm is actually measuring when it goes off.
If it is sounding right now
If your alarm is making a repeating pattern of beeps, not a single chirp, stop reading and act:
- Get everyone, including pets, out of the house into fresh air.
- From outside, call 911 or the fire department.
- Do not go back in to look for the cause, open windows, or turn appliances off on your way. Leave that to the responders, who carry meters.
- If anyone has a headache, nausea, dizziness, weakness or confusion, tell the dispatcher. Those are the common early signs of CO poisoning and are easy to mistake for the flu.
Pressing the silence button does not make the CO go away. If the alarm sounded for real, the gas is still there. See related: How to Respond When Your Carbon Monoxide Alarm Goes Off.
The back label, field by field
Take your alarm off its mounting (most twist off) and look at the back. Every alarm has a label like this one. The details on yours will differ, so read yours, but the fields are the same.
+----------------------------------------------------+
| CARBON MONOXIDE ALARM Battery: sealed |
| Listed to UL 2034 |
| |
| MFD: 2019 FEB 11 REPLACE BY: 2026 FEB |
| (7-year sensor life from date of manufacture) |
| |
| WHAT YOU HEAR WHAT IT MEANS |
| 4 quick beeps, pause, repeat CO DETECTED. Leave |
| now, call 911. |
| 1 chirp every 60 seconds Low battery |
| 5 chirps every 60 seconds END OF LIFE. |
| Replace this alarm. |
| Red light flashing, no sound Normal standby |
| (varies by model) |
| |
| Push TEST monthly. Do not paint. |
+----------------------------------------------------+
Listed to UL 2034. UL 2034 is the safety standard for residential CO alarms. A listing means a testing laboratory checked that this model alarms when it should and stays quiet when it should. Buy only listed alarms.
MFD (manufactured). The date the alarm was made, not the date you installed it. An alarm can sit on a shelf for months before you buy it.
Replace by. CO sensors wear out. Depending on the model, the sensor is rated for somewhere around 5 to 10 years from manufacture, and the label tells you which. After that date, the alarm may no longer detect CO reliably, even though its light still blinks and its test button still beeps. The test button checks the horn and the electronics, not the sensor's ability to smell the gas.
What you hear. This is the key the rest of the card is built on. The pattern above is typical: a CO alarm uses four quick beeps and a pause, repeated. A smoke alarm uses three beeps and a pause. A single chirp at long intervals is a battery or end-of-life signal. Your label gives the exact patterns for your model, and they vary, especially for the chirps.
An alarm, a low battery, and end of life are three different things
| Signal | What is happening | What you do |
|---|---|---|
| Repeating pattern of beeps (typically 4 and a pause) | The sensor has seen enough CO, for long enough, to cross its alarm threshold | Leave, call 911 from outside |
| Single chirp, roughly once a minute | Battery is low (on a replaceable-battery model) | Replace the battery during the day; test |
| Different chirp pattern, or "End" on a display | The sensor has reached the end of its rated life | Replace the whole alarm |
The low-battery chirp has a habit of starting in the middle of the night. That is not bad luck: houses cool overnight, and a weak battery's voltage drops as it gets colder, so it crosses the warning point at 3 a.m. rather than at noon.
Sealed-battery alarms have a battery built in for the life of the alarm. They do not get a new battery. When they chirp, it is time to replace the whole unit.
What the alarm is actually measuring
A CO alarm does not go off the instant it detects any carbon monoxide. Under UL 2034, it is designed to respond to a combination of concentration and time, roughly the way your body accumulates CO. The standard requires an alarm to sound within these windows:
- at 70 parts per million (ppm), within 60 to 240 minutes
- at 150 ppm, within 10 to 50 minutes
- at 400 ppm, within 4 to 15 minutes
It also requires the alarm to stay quiet at low levels such as 30 ppm, so that ordinary brief exposures like a car starting in an attached garage do not set it off.
Two things follow from that.
An alarm that sounds means the CO has been there a while. At moderate levels, the alarm has been counting for an hour or more before it sounds. That is why the right response is to leave, not to investigate.
A quiet alarm does not mean zero CO. It means the level has stayed below what the alarm is built to report. People with heart or lung conditions, infants and pregnant women can be affected at levels a standard alarm will not announce. If anyone in the house feels the symptoms above, especially when a fuel-burning appliance is running, call for help even if the alarm is quiet.
Why a silenced alarm that sounds again is real
Most alarms have a button that silences the horn for a few minutes. If CO is still present when that time is up, the alarm sounds again. That second alarm is the alarm checking again and getting the same answer. It is not a glitch.
Worked example: two nights, two signals
Night one. At about 3 a.m., a homeowner hears a burst of five short chirps from the hallway, then nothing, then another burst about a minute later. No repeating beeps. She takes the alarm down and reads the back label: MFD 2019 FEB, replace by 2026 FEB, sealed battery. It is now October 2026.
From February 2019 to February 2026 is 7 years, the rated life printed on the label. February 2026 to October 2026 is 8 more months. The alarm has been past its replace-by date for about 8 months, and the chirp pattern on her label for end of life matches what she is hearing. This is not an emergency, but it does mean the hallway has had no working CO protection for most of a year. She deactivates the old alarm the way its manual describes (many sealed units have a switch or tab for this), and buys and installs a replacement the next morning. She writes the new alarm's MFD date and replace-by date in her house records.
Night two, a different house. At 11 p.m., the alarm near the bedrooms sounds four quick beeps, a pause, four more. A teenager presses the silence button, assuming a false alarm. Four minutes later it sounds again. This time the parents take everyone outside and call the fire department. The responders' meter reads elevated CO inside, and the source turns out to be the furnace, whose exhaust was not venting properly. The furnace is shut down until a licensed technician repairs it.
What separated the two. The pattern of the sound. Chirps at long intervals are the alarm asking for attention. A repeating pattern of beeps is the alarm reporting gas. And the alarm sounding again after being silenced is the clearest sign there is that the gas is real.
Where alarms belong
The International Residential Code requires CO alarms in homes that have a fuel-burning appliance or an attached garage, placed outside each separate sleeping area in the immediate vicinity of the bedrooms, and inside a bedroom if a fuel-burning appliance is in it or in its attached bathroom (IRC R315 in the 2021 edition, as adopted by your jurisdiction). The U.S. Consumer Product Safety Commission and many fire safety groups also recommend one on every level of the home. State rules differ: California, for example, requires CO alarms in existing homes with a fuel-burning appliance, fireplace or attached garage, while other states require them only in new construction or when a home is sold or rented, and some have no statewide rule. Your local fire department can tell you what applies to your house.
Follow your alarm's manual for exact placement. Most manuals warn against mounting a CO alarm right next to a furnace or water heater, in a garage, in a bathroom, or near a window or vent, because those spots give false readings or miss the air you actually breathe.
How to know your alarms are working
- Press the test button on each one monthly, and confirm the sound.
- Read the replace-by date on each one, and write it in your house records.
- Replace any alarm past its date, even if it still tests.
- If you have both a smoke alarm and a CO alarm, or a combination unit, learn both patterns. See related: What Your Smoke Alarms Need From You.
References
- UL 2034, Standard for Single and Multiple Station Carbon Monoxide Alarms, which sets the response windows above and binds the manufacturer of a listed alarm
- International Residential Code R315 (2021 edition), as adopted by your jurisdiction
- U.S. Consumer Product Safety Commission guidance on carbon monoxide alarms
- See related: Carbon Monoxide Testing + Response Reference
- See related: When to Stop and Call a Licensed Pro Around the House