Carbon Monoxide Alarm Beeping: Alarm, Battery or Fault?

If your carbon monoxide alarm is sounding loudly and repeatedly, it has detected CO. Treat it as real: turn off gas appliances if you can do it safely, open doors and windows, get everyone outside and call the National Gas Emergency Service on 0800 111 999. If the alarm is making a single short chirp every minute or so instead, it is almost always telling you about itself: a low battery, the end of its working life or a fault. The rest of this guide explains how to tell the difference and what happens next.

Alarm or chirp: how to tell the difference

Every make uses its own sound and light patterns, so the label on the back of the alarm or its manual is the final word. In general, though:

• Full alarm. Loud, piercing beeps in a continuous or repeating pattern, often with a flashing red light. This means CO is present. Act immediately.
• Low battery. A single short chirp roughly every 30 to 60 seconds, sometimes with a yellow or amber light flash.
• End of life. A chirp pattern, often two or three chirps at intervals, that starts when the sensor reaches the end of its design life. Many alarms have a sealed battery and are meant to be replaced as a whole unit at this point.
• Fault. Another chirp or light pattern meaning the alarm can no longer detect CO reliably. Replace it.

If you can't tell which it is, assume it's a real alarm until you've checked. It takes a minute to get outside and read the label; it isn't worth the risk the other way round.

What to do if your CO alarm goes off

After the alarm: what happens with the boiler

The emergency service engineer's job is to make the property safe. If they find a problem with an appliance, they may turn it off and label it as unsafe to use. They don't repair boilers. Before anyone uses the appliance again, a Gas Safe registered engineer needs to find the cause and fix it.

Common causes behind a boiler producing CO include a blocked or damaged flue, a poorly burning or dirty burner, a seal on the casing or flue that has failed, and a boiler that has gone years without a service. Our guide to signs of carbon monoxide from a boiler covers what to look for, and blocked or frozen flues explains one of the more common winter causes.

The autumn switch-on is when many of these faults show up, because the boiler has sat idle all summer. Our boiler repair and boiler service engineers are Gas Safe registered and will check combustion and the flue as part of the job.

Chirping: low battery, end of life or fault

A chirping alarm isn't an emergency, but it does mean you may not be protected. Work through it in this order:

1. Read the label. Most alarms print the chirp patterns and a replace-by date on the back.
2. Check the date. Sealed-battery alarms are usually designed to last 7 or 10 years from when they're first activated. If yours is past that, the chirp is an end-of-life warning and the whole unit needs replacing.
3. Replace the battery if it's a model with a replaceable one, then press and hold the test button.
4. Replace the alarm if it still chirps, if it's past its date, or if the test doesn't sound.

Buying a new one? HSE advises choosing an alarm that meets British Standard EN 50291 and carries a British or European approval mark such as a Kitemark. Our guide to carbon monoxide detectors covers where to fit it and the rules for rented homes.

Can a CO alarm go off for no reason?

Detectors can fail, and an ageing sensor can occasionally misbehave. But you should never decide on the spot that an alarm is false. CO can come from places other than your own boiler: a car left running in an attached garage, a blocked chimney on an open fire or wood burner, fumes drawn in from a neighbour's flue in a converted house or block, or a generator or barbecue used near a window. Only an engineer with the right equipment can rule these out.

If the alarm has gone off more than once and nobody has found a cause, that's a reason for a proper check, not to take the alarm down.

Checking and replacing an alarm

• Press the test button once a month, and before you turn the heating back on each autumn. Our pre-season heating checklist includes this.
• Keep the alarm clear of dust, paint and sprays, and don't put it in a cupboard.
• Have one in every room with a gas or solid fuel appliance, and consider one in bedrooms too.
• If you rent in England, your landlord has had to provide a CO alarm in any room used as living accommodation with a fixed combustion appliance (other than a gas cooker) since 1 October 2022, according to HSE.

If you smell gas at any point, follow our gas leak advice and call 0800 111 999 first. For the boiler check afterwards, our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts.

Frequently Asked Questions