Carbon Monoxide Alarm Beeping or Chirping: What Each Pattern Means

If your carbon monoxide (CO) alarm is sounding a loud, repeating pattern (on many models four short beeps, a pause, then four beeps again), treat it as an emergency: get everyone outside into fresh air now and call 911 (in the UK, 999 if anyone is unwell, and the gas emergency line on 0800 111 999). A single short chirp roughly once a minute is a different signal: on most models it means a low battery, and a chirp that does not stop after a new battery usually means the alarm has reached the end of its life and must be replaced. Patterns differ between makes and models, so check the manual for yours, and if you cannot tell which signal you are hearing, go outside first.

This guide is based on the public sources linked in the text and listed at the end. We have not tested any alarm. It is general safety information, not medical advice: if anyone feels unwell, call 911 (US) or 999 (UK) and see a doctor.

For where to put alarms in the first place, see our CO alarm placement guide. This page is about the sounds: what each one means and what to do next.

This is an emergency: four beeps and a pause

The US Consumer Product Safety Commission (CPSC) is blunt: "Never ignore an alarming CO alarm!" Its carbon monoxide fact sheet says to move outside to fresh air immediately, call emergency services, the fire department or 911, and not to go back in until the responders give permission.

What the emergency sound is depends on the model. Two of the largest US makers describe it the same way:

• First Alert: "4 Beeps and a Pause: EMERGENCY", meaning CO has been detected, so move to fresh air and call 9-1-1 (First Alert).
• Kidde: four short beeps, five seconds of silence, four short beeps (Kidde). Kidde adds that if a digital alarm shows any number, with or without a sound, you should assume it is sensing CO.
• Combination smoke and CO alarms: X-Sense notes that on many US combination alarms, repeated groups of three beeps mean smoke and groups of four mean carbon monoxide (X-Sense). Some alarms also speak ("Warning: carbon monoxide").

Feeling fine does not make it a false alarm: CO has no colour or smell, and the CDC warns that people who are asleep can die before they have symptoms (CDC).

What to do when the alarm sounds

1. Get everyone out into fresh air, including pets, and count heads. Do not stay inside to look for the source.
2. Call for help from outside: 911 or the fire department in the US. In the UK, call the National Gas Helpline on 0800 111 999 (free, 24 hours) if you suspect gas appliances, and 999 if anyone has trouble breathing, sudden confusion, chest or muscle pain or has passed out (NHS).
3. Do not go back in until the emergency responders say it is safe (CPSC). Kidde adds: wait until the home has been aired out and the fire department has given the all clear (Kidde).
4. Do not use the appliance you suspect until a qualified person has checked it. The UK Health and Safety Executive says to switch off the appliance and shut off the gas at the meter if you can do so safely (HSE).
5. If it goes off again, Kidde says that if the alarm reactivates within 24 hours you should repeat the steps and call a qualified appliance technician.

Pressing the silence or reset button stops the noise, not the gas. Use it only once everyone is out or help is on the way.

Every pattern at a glance

These are the signals the manufacturers' own pages describe. Your model may differ: confirm it in the manual (the model number is on the back of the alarm).

Kidde says its alarms cannot detect CO once they enter end-of-life mode (Kidde). An expired alarm that chirps means you have no CO protection in that spot.

Low battery or end of life?

Put in a fresh battery and listen. If the chirping stops, it was the battery. If it carries on, the sensor has reached the end of its life and the alarm needs replacing, not repairing.

How long alarms last depends on the maker:

• CDC: replace your CO detector following the manufacturer's instructions or every 5 years (CDC).
• First Alert: most CO alarms last between 5 and 7 years; its 10-year sealed-battery models last a decade.
• Kidde: since 2013 nearly all Kidde CO alarms have a 10-year life; ones made before 2013 have a 7-year life (Kidde).

So the realistic range is about 5 to 10 years. Look for the manufacture date on the back of the alarm; if it is older than the maker's stated life, replace it even if it is silent. The CPSC notes that newer alarms have an end-of-life signal that tells you when to replace them.

Testing and battery care

• Test monthly and replace batteries every year, says the CPSC (CPSC CO alarms). The CDC suggests checking batteries when you change your clocks in spring and fall. In the US the clocks go back on Sunday 1 November 2026.
• Know what the test button checks. The CPSC says it tests "whether the circuitry is operating correctly, not the accuracy of the sensor". A passing test does not tell you an old sensor still works, which is why the age matters.

Where the carbon monoxide comes from

The CPSC lists malfunctioning fuel-burning appliances (furnaces, ranges, water heaters and room heaters), engine-powered equipment such as portable generators, fireplaces, and charcoal burned indoors. It also warns never to leave a car running in an attached garage, even with the garage door open. The CDC adds grills, camp stoves and other fuel-burning items used indoors.

In the UK, the HSE lists warning signs on gas appliances: yellow or orange flames instead of blue, soot or yellow-brown staining on or around the appliance, pilot lights that keep going out, and more condensation inside windows. The CPSC recommends having the heating system professionally inspected and serviced every year. Generators are a common cause after storms: see our guide to portable generators and carbon monoxide, and for tents and campers, camping heaters and CO.

Symptoms: what the health bodies say

We quote the official lists rather than interpret them:

• CDC: "The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion."
• CPSC: early symptoms of low to moderate exposure "are similar to the flu (but without the fever)".
• NHS: headache, dizziness, feeling or being sick, feeling weak, tiredness and confusion, chest and muscle pain, shortness of breath and loss of vision; symptoms may ease when you leave the building.

If anyone in the home feels unwell, especially if more than one person or a pet does at the same time, get everyone into fresh air and call 911 (US) or 999 (UK). For milder symptoms in the UK, the NHS says to call 111 for advice. See a doctor even if the symptoms ease. The CDC says infants, older people and people with heart disease, anaemia or breathing problems are more likely to become ill.

"CO2 alarm beeping": a common mix-up

Many people search for a "CO2 alarm" when they mean a carbon monoxide alarm. Carbon monoxide (CO) is the poisonous gas from burning fuel that home alarms are built to detect. Carbon dioxide (CO2) is a normal part of the air we breathe out; the CDC's NIOSH guide describes it as a colourless, odourless gas present in normal air at about 300 ppm (NIOSH). If the alarm on your wall says "CO" or "carbon monoxide", everything in this guide applies to it.

US and UK rules

• US: the CPSC recommends CO alarms on each level of the home and outside sleeping areas. Some states and cities set their own CO alarm rules, so check locally.
• England (rented homes): since 1 October 2022, landlords must fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance (gas cookers are excluded), check it works on the first day of a new tenancy, and repair or replace it once told it is faulty (GOV.UK guidance). If you rent and the alarm is chirping, the guidance says to try new batteries first and report it to your landlord if it still does not work.
• UK homes generally: the NHS advises a CO alarm in each room that has an appliance burning gas, oil, coal or wood. The GOV.UK booklet says alarms should comply with British Standard BS 50291.

What to buy, if anything

If the chirp stopped after a new battery, you need nothing more than spare batteries of the type printed on the alarm. If the alarm is past its stated life, shows END, or keeps chirping with a fresh battery, buy a replacement. When you compare alarms, check:

• The life and the date: a stated replacement age (often 7 or 10 years) and a clear manufacture date.
• Power: a sealed 10-year battery, replaceable batteries, or plug-in or hardwired with battery backup (the CPSC and CDC both recommend battery or battery-backup alarms).
• The standard: in the US, the CPSC lists UL 2034 as the voluntary standard for CO alarms (CPSC); in the UK, the GOV.UK booklet names BS 50291.
• A digital display and interconnection, if you want to see CO levels or have every alarm sound together. With a combination smoke and CO alarm, learn which pattern means which gas.

Last checked: 26 October 2026. Sources: CPSC carbon monoxide fact sheet; CPSC CO alarms; CDC carbon monoxide basics; NHS carbon monoxide poisoning; HSE gas safety: carbon monoxide; GOV.UK smoke and CO alarm guidance; Kidde beeping and chirping; Kidde replacement; Kidde alarm action; First Alert beep meanings; X-Sense beeping guide; NIOSH carbon dioxide.