Carbon monoxide (CO) has no smell or colour. If an alarm sounds, treat it as a real warning even if you feel well and the boiler appears to be working. The first call is to the emergency service from outside, not to a routine repair appointment.
Stop using fuel-burning appliances if you can do so on your way out, then leave the building immediately. From outside, call the National Gas Emergency Service on 0800 111 999. Do not go back in until the emergency service says it is safe.
What to do in order
Leave with children, visitors and pets. Do not delay to find the source or collect belongings.
Report the alarm to the National Gas Emergency Service on 0800 111 999 and follow its instructions.
Tell NHS 111 if you may have been exposed or feel unwell. Call 999 for severe symptoms such as breathing difficulty, sudden confusion or collapse.
Do not restart an appliance or re-enter the property until the emergency service advises it is safe.
Do not try to prove it was a false alarm
- Do not silence or reset the alarm and stay inside to see whether it sounds again
- Do not assume a lack of smell means there is no danger
- Do not open a boiler casing, flue or fuel-burning appliance to investigate
- Do not mistake a possible low-battery chirp for a safety clearance while the alarm is sounding
An alarm's low-battery signal and a CO alert can sound different, but the pattern depends on its model. Check the manufacturer's instructions only after everyone is safe and the emergency response has dealt with the alert.
What happens after the emergency response?
The emergency service may isolate an appliance or supply. Ask what was made safe and what must stay off. If a gas appliance needs inspection or repair, use a Gas Safe registered engineer; other fuel-burning equipment needs an appropriately qualified specialist. Do not use the appliance again merely because the alarm has stopped.
For the official response steps, read UK Health Security Agency guidance and NHS advice on symptoms. Once the property is declared safe, our boiler service guide explains what a routine inspection covers; for a gas appliance fault, request a heating engineer.