Four fast beeps, a pause, then four more. On a First Alert carbon monoxide alarm or combination alarm, that is the pattern that means carbon monoxide has been detected. It is not a maintenance chirp and it is not a battery warning. It is the alarm doing the job you bought it for.
Get out. Get fresh air. Call the emergency services. Do those three things before you look at the unit, read the manual, or search for an explanation.
What this signal means
First Alert documents this pattern directly. Its safety guidance for carbon monoxide alarms lists “4 Beeps and a Pause” as an emergency, meaning carbon monoxide has been detected and you should move to fresh air and call the emergency services. The SMCO500V combination alarm manual describes the same event as four fast beeps with a pause, repeated, announced by voice as an evacuation warning for carbon monoxide.
The Firemen’s Association of the State of New York teaches the same shorthand: smoke alarms sound three beeps, carbon monoxide alarms sound four. It is a convention used across the industry rather than a legal requirement, so the manual for your exact model is always the final word.
One important exception: the test sequence can also produce a four-beep group. That is why the first question is not “how many beeps” but “was anybody holding the test button”. If nobody was, treat it as real.
What to check first
Not the alarm. The people.
- Move everyone to fresh air. Take the stairs, not the lift if you are in a block, and take pets with you.
- Call the emergency services and say a carbon monoxide alarm is sounding.
- Count heads. Anyone with a headache, dizziness, nausea, vomiting, weakness, sleepiness, or confusion needs to be assessed by a medical professional.
- Do not re-enter to open windows, find the source, or fetch belongings.
- Leave the alarm sounding. A silenced alarm cannot warn anyone who goes back in.
- Wait for responders to clear the property before you touch the unit at all.
The Consumer Product Safety Commission calls carbon monoxide the “Invisible Killer” for a reason: it is colourless and odourless, so your senses cannot confirm or rule it out. Neither can a quick look at your boiler.
Causes, from most to least likely
| Cause | How to confirm | Fix |
|---|---|---|
| Blocked or cracked flue on a boiler, fire, or water heater | Technician inspection, or a repeat alarm on cold mornings | Qualified technician repairs the flue and re-tests appliances |
| Running a generator, charcoal grill, or camp stove indoors | Obvious from where the appliance is being used | Stop using it indoors and move it outside, well away from openings |
| A car left running in an attached garage | Vehicle running with the garage door shut or partly shut | Never idle a vehicle in an attached garage |
| Blocked or reversed ventilation in a sealed room | Weak draught at vents, condensation, technician test | Improve ventilation and service the appliance |
| A back-drafting wood stove or open fire | Smoke smell indoors or a technician reading | Service the chimney and check pressure balance |
| A faulty or expired alarm sensor | Manufacture date seven or more years old | Replace the alarm with a current model |
| Low battery or a power fault | One short chirp every 30 to 60 seconds, not a four-beep repeat | Fit the correct battery and reset the unit |
Note the shape of that table. The most likely causes are real sources of carbon monoxide, not device faults. Starting with the alarm is the classic mistake.
When it is a real alarm
Four fast beeps and a pause is a real alarm until a qualified person tells you otherwise. Two more signals confirm it:
- A red LED flashing in time with the horn rather than a slow standby blink.
- A digital display on models that have one, showing a carbon monoxide reading instead of a blank or zero.
If the unit talks, voice models will name the location, for example announcing an evacuation warning for carbon monoxide in a named room. Believe it.
Do not test the air yourself with a naked flame, a candle, or by breathing deeply to see how you feel. None of those are tests, and one of them can hurt you.
When to replace the unit instead of repairing it
Replace the alarm when any of the following applies:
- The manufacture date shows seven or more years of service. First Alert states that most carbon monoxide alarms should be replaced after seven years, and that all carbon monoxide detectors should be replaced after seven years even if the battery is still good.
- The unit sounds a full alarm pattern after the property has been cleared and checked, with no source found.
- It fails a test, or the test tone is weak, crackly, or intermittent.
- The housing is cracked, discoloured, or has been painted or sprayed.
- The date label is missing or unreadable.
Do not treat a passing test as proof that the sensor works. The test button confirms power and the sounder. The sensing element is what ages, and that is the part you cannot check at home.
Prevention
- Install carbon monoxide alarms on every level of the home and outside each sleeping area. The CPSC recommends interconnected alarms so that when one sounds, they all sound.
- Keep alarms at least 15 feet from a furnace or other fuel-burning source where space allows, and out of bathrooms and dead-air corners.
- Have fuel-burning appliances, flues, and chimneys serviced by a qualified technician on the manufacturer’s schedule.
- Never run a generator, charcoal grill, or portable camping stove inside a home, garage, basement, or tent.
- Never idle a car in an attached garage, even with the door open.
- Test alarms monthly and replace backup batteries with the correct type.
- Replace the whole alarm on schedule and label the install date on the unit.