Carbon monoxide alarm beeping? What each sound means and what to do
A carbon monoxide alarm that sounds loudly and keeps repeating is warning of CO in the air. Treat it as real: open doors and windows, turn off the gas if you can, get everyone outside and call 0800 111 999. Short beeps about once a minute are different: a low battery, a fault or an alarm at the end of its life. Here is how to tell them apart on an Ajax FireProtect 2.
Loud alarm or a single chirp: which is it?
Gas Safe Register says an alarm sounding continuously could mean carbon monoxide is present, while other beeping, such as a low battery or a fault, depends on the model, so check the maker's instructions.
- Loud, fast and repeating: a CO alarm. Act first.
- One to three short beeps, then about a minute of quiet: a maintenance warning. The coloured light tells you which.
CO has no colour, smell or taste (Gas Safe Register). NHS-listed symptoms include headache, dizziness, feeling sick, tiredness, confusion and shortness of breath.
What should you do when a CO alarm goes off?
- Turn off the gas supply and open doors and windows if it is safe to (Gas Safe Register).
- Get everyone out and stay out.
- If a gas appliance could be the cause, call the National Gas Helpline on 0800 111 999, free and open 24 hours (NHS).
- Get medical advice: NHS 111 if you think you have symptoms, 999 if anyone has difficulty breathing, sudden confusion, chest pain or loses consciousness.
- Don't use the fuel or gas appliance again until a registered engineer has checked it, as Ajax's manual advises.
Test/Mute on a FireProtect 2 buys only 10 minutes of quiet (Ajax). If CO remains, the alarm restarts, and above 300 ppm it refuses to mute, giving a short, low beep.
What does each FireProtect 2 sound mean?
From Ajax's manuals for the FireProtect 2 (CO) and mains AC models. Red is an alarm, yellow maintenance, green power.
| Sound | Light | Meaning | Action |
|---|---|---|---|
| Three beeps, repeating every 3 seconds | Red, three flashes each time | Dangerous CO level | Follow the steps above |
| A different alarm pattern | Red (continuous on the AC Heat/Smoke/CO) | Smoke or heat, on combined models | Get out and call 999 |
| One beep a minute | Yellow, one flash a minute | Low battery (AC: backup battery) | See below |
| Two beeps a minute | Yellow, two flashes a minute | Malfunction | Check the Ajax app |
| Three beeps a minute | Yellow, three flashes a minute | End of life, over 10 years old | Replace the detector |
| Five beeps, then three longer | Red, five flashes then three slower | Self-test | None |
| None | Green, once a minute | Battery models: power fine. AC: on backup, mains lost | AC: check the mains |
Why is my CO alarm chirping every minute?
One beep and one yellow flash a minute is the low-battery warning. Ajax says the detector keeps working for about another month afterwards, so it isn't an emergency, but don't leave it:
- RB (replaceable batteries): fit two new CR123A cells the right way round; Ajax says never fit used or damaged ones.
- SB (sealed batteries): replace the whole detector. Ajax rates the SB (CO) battery at 10 years with its Battery Life Optimisation setting on.
- AC (mains): the backup battery is low; Ajax says replace the detector once it is discharged.
Two yellow flashes means a fault, which a hub shows in red in the Ajax app; Ajax asks you to contact its support if a repair is needed. Test/Mute silences a battery, fault or end-of-life warning for 12 hours, then it comes back.
How long does a carbon monoxide alarm last?
Gas Safe Register says most CO alarms last five to ten years. Ajax rates the FireProtect 2 CO models at 10 years, prints the end-of-life date on the detector, and then gives three beeps and three yellow flashes a minute because the sensor may have lost sensitivity. Ajax recommends a monthly self-test: hold Test/Mute for about 1.5 seconds.
Who deals with a beeping CO alarm in a rented home?
In England, the Smoke and Carbon Monoxide Alarm (England) Regulations 2015, as amended in 2022, require private and social landlords to fit a CO alarm in any room used as living accommodation with a fixed combustion appliance, such as a gas or oil boiler or a log-burning stove, but not a gas cooker. GOV.UK guidance says landlords must repair or replace an alarm once told it is faulty, and councils can fine up to £5,000 per failed remedial notice. Tenants are advised to change batteries and report the alarm if that fails; a sealed SB model avoids this.
In Scotland, all homes need a CO detector in any room with a carbon-fuelled appliance such as a boiler, open fire or wood burner (gov.scot). In Wales, Welsh Government guidance says landlords must have one in any room with a gas, oil or solid fuel appliance they installed.
Which Ajax CO alarm, and what does a hub add?
Every FireProtect 2 works alone with its built-in siren (85 dB at 3 m), but Ajax says a standalone detector sends no notifications to the Ajax apps. Paired with a hub, alarms, faults, low battery and end-of-life warnings reach your phone and can go to a monitoring station. When one detector alarms, the others sound within 20 seconds, all showing the type of alarm found, so CO isn't mistaken for fire.
| Model | Senses | Power | Inc VAT |
|---|---|---|---|
| FireProtect 2 AC (CO) mains alarm | CO | Mains plus backup | £95.76 |
| FireProtect 2 SB (CO) sealed-battery alarm | CO | Sealed batteries | £147.69 |
| FireProtect 2 SB (Heat/CO) | Heat, CO | Sealed batteries | £150.00 |
| FireProtect 2 AC (Heat/Smoke/CO) | Smoke, heat, CO | Mains plus backup | £105.00 |
| FireProtect 2 RB (Heat/Smoke/CO), replaceable batteries | Smoke, heat, CO | 2 x CR123A | £150.00 |
Hubs include the Hub (4G) Jeweller alarm panel from £114.23, the Hub 2 (2G) wireless panel at £161.53 and the Hub 2 Plus at £268.10; Ajax says Hub (4G) needs OS Malevich 2.28 or later firmware for these detectors. Prices include VAT, delivery is extra, and prices can change. The same hub can also run an Ajax wireless burglar alarm. See the Ajax wireless fire alarm range, our standalone smoke and heat alarms or everything we stock from Ajax.
Our take
Treat a loud, repeating CO alarm as real every time, and learn the chirps before you need them: one yellow flash is the battery, two a fault, three end of life. A sealed SB model removes battery changes, and in a rented property a hub means the warning reaches you as well as the tenant. Zenith Security Co. has supplied security equipment since 2012. We supply Ajax alarms rather than fit them, and we can recommend a qualified installer depending on the job and area; the AC versions need wiring to the mains.
Not sure which fits? Call 01902 902251, send us a message or visit our Wolverhampton trade counter. We ship nationally to England, Scotland and Wales.
CO alarm beeping: quick questions
Why is my carbon monoxide alarm beeping three times?
On a FireProtect 2, three beeps with a red light every 3 seconds is a CO alarm: get out. Three beeps with a yellow light once a minute means it is past its 10-year life.
Does a smoke alarm detect carbon monoxide?
No, says Gas Safe Register. Combined models such as the FireProtect 2 (Heat/Smoke/CO) have a separate CO sensor.
Which standard should a CO alarm meet?
Gas Safe Register recommends a Kitemark to BS EN 50291-1 for homes. Ajax lists EN 50291-1 on its FireProtect 2 CO models and a BSI Kitemark (KM 797661) on the AC and SB versions.
Can I take the battery out to stop the beeping?
SB and AC batteries are sealed in, and removing an RB's batteries switches off your protection. Fit new batteries on an RB, or replace the detector.
Source: Ajax Systems blog, 11 June 2025, checked against the Ajax manuals (UK region) for the FireProtect 2 (CO), AC (CO) and AC (Heat/Smoke/CO) and its RB battery guide, with Gas Safe Register, the NHS, GOV.UK, gov.scot and the Welsh Government. Device figures are Ajax's own.
