Carbon monoxide alarm has gone off — what to do
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Carbon monoxide is odourless and colourless. If the alarm is sounding and anyone feels unwell, treat this as a gas emergency — call 999 immediately.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
If your carbon monoxide alarm sounds, act immediately. Do not try to identify the source. Get everyone (including pets) out of the property straight away. Do not go back inside.
Once outside, call the National Gas Emergency number: 0800 111 999 (free, 24 hours). If anyone is feeling unwell, dizzy, has a headache, or is nauseous, call 999 and tell them CO poisoning is suspected. Go to hospital even if symptoms are mild — CO poisoning can cause delayed neurological damage.
Do not re-enter the property until the emergency services or a Gas Emergency Responder (GER) has attended, found and isolated the source, and confirmed it is safe. The National Gas Emergency service will attend within 1–2 hours.
Common sources of CO in a home: a gas boiler with a cracked heat exchanger, a blocked or damaged flue, a gas cooker or hob with poor combustion, a gas fire with a blocked chimney, or a portable generator or barbecue used indoors.
After the emergency has been resolved, a Gas Safe registered engineer must inspect and repair the appliance before it is used again. Do not simply reset the alarm and restart the boiler.
If your alarm was false (battery low, steam, dust near the sensor), replace the battery or clean the detector. CO alarms have a lifespan of 5–7 years — replace them when expired. British Standard BS EN 50291 specifies that alarms should be fitted on each floor, ideally in each room with a gas appliance.
This job needs a qualified engineer — post it now
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Was this guide helpful?
Frequently asked questions
What should I do if a carbon monoxide alarm sounds?
Get everyone out of the property immediately — do not stop to investigate. Once outside, call the National Gas Emergency number: 0800 111 999 (free, 24/7). If anyone feels unwell, dizzy, or has a headache, call 999 and tell them CO poisoning is suspected. Do not re-enter until emergency services confirm it is safe.
Can a CO alarm give a false alarm?
Yes — CO alarms can false-alarm due to low battery, steam, dust near the sensor, or if the alarm has reached the end of its lifespan (usually 5–7 years). However, always treat a sounding CO alarm as genuine until confirmed otherwise. Do not assume it is a false alarm and ignore it.
Where should CO alarms be fitted?
On each floor of the property, at head height, within 3 metres of any gas, oil, or solid fuel appliance. Do not fit them in kitchens directly above cookers or in damp areas. Prioritise bedrooms nearest the boiler — CO poisoning most commonly happens at night when people are asleep.
How do I know if my CO alarm is still working?
Test the alarm monthly using the test button. Most alarms have a lifespan of 5–7 years — check the expiry date on the label. An end-of-life alert (usually a different chirp pattern from the low battery alert) means the unit must be replaced, even if the battery is fresh.