Short answer
Four quick beeps, a pause, and repeat is a carbon monoxide alarm: get everyone outside and call 911. A single chirp about once a minute usually means a low battery. Other chirp patterns signal end of life or a fault, as shown on the unit's label. An end-of-life signal means replacing the whole detector.
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What does it mean when a carbon monoxide detector beeps?
It depends on the pattern. Four quick beeps followed by a pause, repeating, is the standard carbon monoxide alarm and means the device has detected a dangerous level of CO. A single chirp about once a minute usually means a low battery. Other chirp patterns signal end of life or a fault, as listed on the unit's label.
Because the emergency signal and the maintenance signal can sound similar in a quiet house at night, the first step is to listen through a full cycle. Repeated groups of four beeps are not a battery problem. A lone chirp with a long silence after it is. Combination smoke and CO alarms use a different pattern for each hazard, and many also show a light or play a voice message naming the hazard.
Treat any uncertain pattern as an alarm until it is confirmed otherwise. Silencing the device and going back to sleep is the one response that cannot be undone.
What should you do if a carbon monoxide alarm goes off?
Move everyone, including pets, outside into fresh air immediately, then call 911 or the fire department from outside. Do not stay to look for the source, and do not go back in until emergency responders say it is safe. Anyone with a headache, dizziness, nausea, or confusion should get medical attention right away.
Carbon monoxide has no smell or color, so the absence of anything unusual in the house is not evidence of a false alarm. Symptoms of exposure resemble the flu and can affect everyone in the home at once, which is part of why the gas is dangerous during sleep. After responders clear the house, a qualified technician should inspect fuel-burning appliances, vents, and chimneys before they are used again.
Why does a carbon monoxide detector chirp?
A chirp, as opposed to the four-beep alarm, is a maintenance signal. The usual causes are a low or wrong battery, a unit that has reached the end of its service life, or an internal fault. Hardwired CO alarms also chirp when their backup battery weakens. The manufacturer's label or manual lists which pattern means which condition.
A low battery is the common cause, and replacing it with the type the label specifies usually stops the chirp. If the chirp continues, some units need a reset by holding the test or silence button; the manual gives the steps. A chirp after a fresh battery and a reset often means the device has reached the end of its sensor life. As with smoke alarms, chirps on CO alarms often start at night, when cooler temperatures lower a weak battery's voltage.
What does an end-of-life signal on a CO detector mean?
An end-of-life signal means the carbon monoxide sensor has reached the service life its manufacturer designed for, and the whole unit must be replaced. CO sensors degrade with age, and the length of that life varies by model. The replacement date is usually printed on the unit, and many models chirp or display a message when it arrives.
A CO alarm that has aged out may still respond to its test button, because on many models the button tests the horn and electronics, not the sensor's response to gas. That makes the printed date the dependable guide. Some models let the end-of-life chirp be silenced temporarily so there is time to get a replacement; that delay should be short. A replacement is also a good moment to check the location, which our guide to where carbon monoxide detectors should go covers.
Can a carbon monoxide alarm go off with no CO present?
It can happen, but the household cannot confirm it from inside. Aging sensors, very high humidity, and some cleaning products or aerosols can disturb a CO sensor. Because the gas has no smell or color, only responders with gas meters can confirm whether CO is present, so every four-beep alarm should be treated as real until they do.
If responders find no carbon monoxide and the alarm repeats, the unit's age and location are the first things to check. A CO alarm right beside a fuel-burning appliance, in a humid bathroom, or in a garage may react to brief, normal conditions. On a monitored system, a CO alarm is handled as a life-safety signal according to the procedure agreed for the property, as described on our page about smoke, heat, and CO monitoring. For how CO alarms fit alongside smoke protection, see our overview of smoke and carbon monoxide detectors.