Carbon Monoxide Alarm Going Off: What to Do Now
Updated September 18, 2026
Can I do this myself?
Emergency
This is an emergency, not a DIY repair. Get everyone into fresh air and call your emergency number; never silence the alarm and go back inside.
If your carbon monoxide (CO) alarm is sounding its full alarm, get everyone outside into fresh air now, then call your local emergency number such as 911, 112 or 999. Read the rest once you are safe.
Carbon monoxide is a gas with no colour, smell or taste, produced when fuels like natural gas, LPG, oil, kerosene, wood, charcoal and petrol burn without enough air. You can’t sense it, which is why the alarm matters. It stops the blood carrying oxygen, and exposure can cause serious illness or death, sometimes within minutes at high levels. People who are asleep, young children, older adults, pregnant people and those with heart or lung conditions are especially at risk.
Never assume the alarm is faulty, and never silence it and go back inside. CO levels can be high even when everyone feels fine. Only a qualified responder or technician can confirm it is safe. After the emergency, the fuel-burning appliance that caused it needs to be inspected and repaired before it is used again.
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Signs of the problem
- The CO alarm is sounding a loud, repeating pattern (on many models, groups of four beeps with a pause)
- Headache, dizziness, nausea or vomiting, often in more than one person or pets
- Weakness, tiredness, confusion or blurred vision
- Shortness of breath or chest pain
- Symptoms that ease when you go outside and return when you come back
- Signs from appliances: yellow or orange flames instead of blue, soot, or heavy condensation on windows
Common causes
Faulty or poorly maintained fuel-burning appliance
Most commonGas or oil furnaces and boilers, gas water heaters and geysers, gas stoves and ovens, and gas dryers can produce CO when burners are dirty, damaged or badly adjusted.
Blocked flue, chimney or vent
Most commonNests, debris, snow or a disconnected flue pipe can stop exhaust leaving the building, so it spills back into living spaces.
Fuel burning in an enclosed space
SometimesRunning a generator, car or petrol engine in or near the home or attached garage, using charcoal grills or angithis indoors, or unvented kerosene or gas heaters in closed rooms can quickly build up dangerous CO levels.
Gas geyser in a poorly ventilated bathroom
SometimesGas water heaters installed inside small bathrooms without proper venting are a known CO hazard, especially with doors and windows closed.
Alarm fault or end of life
Less commonCO alarms have a limited life and eventually signal end of life with a distinct chirp or pattern. But a full alarm pattern should always be treated as real until responders say otherwise.
Stay safe
- Critical:Get everyone, including pets, outside into fresh air immediately. Don’t stop to investigate.
- Critical:Call your local emergency number such as 911, 112 or 999 from outside, especially if anyone has symptoms.
- Critical:Don’t silence or reset the alarm and go back inside. Don’t re-enter until responders say it is safe.
- If it is quick and on your way out, switch off fuel-burning appliances and open doors, but never delay leaving to do so.
- Seek medical help for anyone with symptoms, even if they improve outside. Tell medical staff it may be CO exposure.
What to do right now
- 1
Get everyone outside into fresh air
Leave the building immediately with everyone in it, including pets. Wake anyone who is asleep. Check that everyone is accounted for once you are outside.
Warning: Don’t spend time looking for the source. CO can make people confused and weak, making it harder to leave the longer you stay.
- 2
Open doors and turn off appliances only on your way out
If it takes no extra time, open the door as you leave and turn off a gas stove or heater you are passing. Don’t go back in to ventilate or to switch anything off.
- 3
Call your emergency number
From outside, call your local emergency number such as 911, 112 or 999. Tell them your CO alarm is going off and whether anyone has symptoms. Some gas utilities also have an emergency line for CO; follow their advice.
- 4
Get medical help for anyone with symptoms
Headache, dizziness, nausea, confusion, chest pain or drowsiness can all be CO poisoning. Anyone with symptoms needs medical assessment, even if they feel better in fresh air. Tell responders how long you were exposed.
- 5
Tell responders which appliances you have
List fuel-burning appliances in the home: furnace or boiler, gas water heater or geyser, gas stove or oven, fireplace or wood stove, kerosene or gas heaters, gas dryer, generator, and any car in an attached garage. Mention anything recently serviced, installed or used unusually.
- 6
Stay out until you are told it is safe
Wait outside, or with a neighbour, until the fire service, gas utility or a qualified technician has checked CO levels and says you can return. Don’t go back in because the alarm has stopped; that doesn’t mean the CO has gone.
Warning: Never silence the alarm and go back to bed. This is how many CO poisonings happen.
- 7
Have the source repaired before reuse
The appliance, flue or chimney that caused the CO must be inspected and repaired by a qualified technician before you use it again. Keep it off until then, even if you are cold.
- 8
Know the difference between an alarm and a chirp
Once everyone is safe, check your alarm’s manual. On many CO alarms, the emergency alarm is a loud repeating pattern (often four beeps then a pause), while a single short chirp every 30 to 60 seconds means low battery, and a different chirp pattern means end of life. Patterns vary by model. If in doubt, treat it as an alarm and get out.
Before you go back in
- Everyone is out in fresh air and anyone with symptoms has medical help
- Emergency services or your gas utility have checked the home and cleared it
- The appliance or vent that caused the CO has been inspected and repaired
- CO alarms are working and tested after the event
When to stop and call a professional
- Any full CO alarm: call your emergency number from outside
- Anyone has symptoms of CO poisoning, even mild ones
- The alarm sounds again after you were told it was safe
- A fuel-burning appliance shows yellow or orange flames, soot, or spillage around the flue
- Any inspection, repair or adjustment of furnaces, boilers, water heaters, flues or chimneys
How to prevent it
- Have fuel-burning appliances, flues and chimneys serviced by a qualified technician every year.
- Install CO alarms on every level of the home and near sleeping areas, and test them monthly.
- Replace CO alarms at the end of their rated life, shown on the device or in its manual.
- Never run generators, cars, charcoal grills or angithis, or unvented heaters indoors, in garages or near windows.
- Keep vents, flues and chimneys clear, and make sure gas geysers are installed with proper ventilation.
Frequently asked questions
What should I do if my carbon monoxide alarm goes off?
Get everyone outside into fresh air right away, call your local emergency number from outside, and don’t go back in until responders say it is safe. Get medical help for anyone with symptoms.
What are the symptoms of carbon monoxide poisoning?
Headache, dizziness, nausea, vomiting, weakness, confusion, blurred vision, shortness of breath and chest pain. Symptoms can feel like flu without a fever, and often affect several people or pets at once.
How do I tell a low battery chirp from a real alarm?
On many models a low battery is a single short chirp every 30 to 60 seconds, while a real alarm is a loud repeating pattern, often four beeps then a pause. Patterns vary, so check your manual. If you aren’t sure, get out and call for help.
Can I just reset the alarm if everyone feels fine?
No. You can’t see or smell CO, and people often feel fine at first. Resetting the alarm and staying inside is dangerous. Leave, call for help and let responders check.
Where should CO alarms be installed?
On every level of the home, near sleeping areas, and in rooms with fuel-burning appliances, following the manufacturer’s placement instructions and any local rules.
This guide is general information, not professional advice. Homes differ, so follow your product manuals and local codes. If anything looks different from what's described, or you feel unsure, stop and call a qualified professional. In an emergency, leave the area and call your local emergency number.
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