Carbon Monoxide Alarm Going 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. You cannot smell or see it. Never ignore a CO alarm, even if you feel fine.
A carbon monoxide alarm going off is one of the most serious emergencies you can face in your home. CO is a colourless, odourless gas produced by faulty or poorly maintained gas appliances — typically boilers, gas fires, and cookers — and it's lethal in high concentrations. Most UK homes have a CO alarm fitted as standard, and when it sounds, it's doing exactly what it should: alerting you to a potentially life-threatening hazard. This guide walks you through the immediate steps to take, how to get professional help, and what happens next to keep your family safe. If your alarm has sounded, treat it as a genuine emergency and act without delay — calling the Gas Emergency Service and a Gas Safe engineer is not optional.
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Get everyone (including pets) out of the building immediately. Do not stop to collect belongings.
Leave doors and windows open as you leave to help ventilate the property.
Once outside in fresh air, call 999 if anyone feels unwell — symptoms of CO poisoning include headache, dizziness, nausea, shortness of breath, and confusion.
Call the National Gas Emergency Service on 0800 111 999 to report a potential CO incident.
Do not go back inside until emergency services or a Gas Safe engineer has declared it safe.
Seek medical attention even if you feel fine — CO can cause delayed symptoms. Tell A&E you may have been exposed to carbon monoxide.
Have all gas appliances (boiler, gas fire, cooker) inspected by a Gas Safe engineer before using them again. The most common causes are a faulty or poorly maintained boiler, blocked flue, or cracked heat exchanger.
Test your CO alarm monthly by pressing the test button. Replace the battery annually and replace the whole alarm every 7–10 years.
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Frequently asked questions
What's the difference between a CO alarm and a smoke alarm?
A smoke alarm detects particles from fire, whilst a CO alarm detects carbon monoxide gas using a chemical sensor. They're separate devices designed to protect against different hazards, and most homes need both. A CO alarm is usually yellow or white, whereas a smoke alarm is typically white with a red test button.
Can I just open a window instead of leaving the house?
No — you must leave the building immediately and get outside into fresh air. Opening windows alone won't remove CO quickly enough, and you could lose consciousness before you realise how serious it is. Get everyone out first, then ventilate the property as you leave.
How long does it take for CO poisoning symptoms to appear?
Symptoms can develop within minutes at high concentrations, or over hours at lower levels. They're often mistaken for flu — headache, nausea, and dizziness are typical early signs. This is why you must seek medical attention even if you feel fine; delayed symptoms can emerge days later.
What if my CO alarm keeps going off after the boiler has been checked?
A Gas Safe engineer should identify the fault during inspection, but if the alarm continues to trigger, have it tested by the engineer again and check the alarm itself isn't faulty or reaching end of life. CO alarms should be replaced every 7–10 years, so age could be the issue — but never assume the alarm is wrong; always treat it as a genuine warning.
Can I stay in a room away from the boiler if the alarm goes off?
No — carbon monoxide spreads throughout a property via air circulation and can accumulate in any room. You cannot isolate yourself from it by staying in a different part of the house. Always evacuate completely and wait for emergency services or a Gas Safe engineer to declare the building safe.
Do I need to replace my boiler if the alarm has gone off?
Not necessarily — a Gas Safe engineer will diagnose the cause first, which could be a blocked flue, poor maintenance, or a cracked heat exchanger. Sometimes a repair is sufficient, but if the boiler is old or badly damaged, replacement may be recommended for safety.