What to Do If a Carbon Monoxide Alarm Goes Off

It is a winter evening and your family is watching TV when the CO alarm in the hallway starts blaring, loud, repeating beeps that are nothing like the quiet chirp you heard last month. Your first thought: "Is this real or is the battery dying again?" The safest answer is always the same: treat it as real first. CO is invisible, odorless, and can overwhelm people quickly, especially during sleep. Your goal is simple: get everyone into fresh air and get the home checked before you go back in. This page gives you a step-by-step emergency response, explains common scenarios, and links to troubleshooting guides for after the crisis.

In 60 Seconds

• Leave immediately with everyone (including pets). Do not search for the source indoors.
• Call emergency services from outside, fire department or your building's emergency contact.
• Do not re-enter until professionals confirm the building is safe.
• If anyone has symptoms (headache, dizziness, nausea, confusion), seek medical help right away.
• After clearance, fix the root cause, inspect fuel-burning appliances, vents, and flues before relying on "reset" alone.

First: Alarm vs Chirp, Know the Difference

People often say their "CO detector alarm is going off" when the unit is chirping for maintenance (battery, end-of-life, fault). The safest approach:

• Loud, repeating alarm (or a display showing CO/PPM) = treat as a CO event. Evacuate.
• Single chirp every 30-60 seconds (often with a flashing light) = usually battery, fault, or end-of-life.

If you are not sure which one you are hearing, assume the worst and evacuate. For chirp patterns and common fixes, see carbon monoxide detector beeping.

Step-by-Step Emergency Response

What Happens When Responders Arrive

The page you are reading ends, for most guides, at "wait outside". That leaves you in a parking lot with no idea what the next hour looks like, which is the point at which people talk themselves into going back in. Here is the shape of it, with the caveat that departments run their own procedures and yours may differ in the details.

They arrive with calibrated instruments rather than opinions. Crews carry gas meters, usually a personal monitor each plus a handheld unit, and they take readings in sequence: at the door before they enter, then room by room, then close to each fuel-burning appliance. Comparing those readings is how they locate a source, because the number climbs as they get near it. A reading that is high at the water heater and near zero at the far bedroom tells them something a single whole-house number cannot.

Expect them to ventilate, and expect that to take time. Expect them to shut off the gas supply to a suspect appliance, and in many jurisdictions to red-tag it, which is a physical tag meaning the appliance stays out of service until a qualified technician clears it. A red tag is not a suggestion, and removing one to get hot water back is how a second incident happens.

The last part is the part worth insisting on. Clearance should come from a meter, not from a conversation. If nobody has read you a number and someone tells you it is probably fine, ask what the readings were and where they were taken. That question is reasonable, responders answer it every day, and it is the difference between a building that has been tested and a building that has been looked at.

What They Will Ask You, and What to Have Ready

Standing outside waiting is the one point in this whole sequence where you have nothing to do, and there is something. The crew will want a short history of the building before they go in, because it tells them where to point the meter first. Work through this in your head, or on your phone, while you wait:

• What burns fuel in there, and where each one is. Furnace, boiler, water heater, range, fireplace, wall heater, gas dryer. Basement, closet, roof space. This is the single most useful thing you can give them, because it is the list they would otherwise build by walking the building.
• When each was last serviced, even approximately. "The furnace was done in October, the water heater has never been touched since we moved in" is a better answer than a shrug and it narrows the search.
• Whether anyone was asleep, and where. It changes their urgency and it changes which rooms they read first.
• How long the alarm had been sounding before you left, and whether it had done anything unusual in the days before, such as chirping and stopping.
• The alarm's manufacture date, which is printed on the back of the unit. You will not have it in the parking lot, so this is the one to look up beforehand rather than during. An expired sensor changes what the activation meant.
• Anything that changed recently. New appliance, new tenant, building work, a blocked flue you noticed, insulation or windows done over the summer. A building that used to be fine and is not any more usually changed.

None of this is a test you can fail. A crew will find the source without it. It gets them there faster, and on a night when the answer is a red tag on a water heater rather than a nothing-found, faster matters.

After Clearance: Fix the Cause

CO alarms are not the "problem", they are the warning. Once responders say it is safe to return, the next job is prevention. Common next steps include:

• Inspection of furnaces, boilers, water heaters, fireplaces, gas stoves, and any other fuel-burning equipment.
• Checking vents, flues, chimneys, and exhaust paths for blockages and backdrafting.
• Reviewing generator and vehicle practices, never run a generator indoors or near windows; never idle a car in an attached garage.
• Replacing any alarm that is expired or malfunctioning. See how long CO detectors last.

For a broader prevention plan, see prevention of carbon monoxide poisoning.

Common Scenarios

"The alarm stopped after a few minutes, was it a false alarm?"

Not necessarily. CO levels can rise and fall with appliance cycles, wind, and ventilation changes. A short event is still a warning that something can produce dangerous CO under certain conditions. Treat it seriously and get an inspection.

"It is the middle of the night, do I really have to leave?"

Yes. Nighttime is high-risk because CO can affect judgment and wakefulness before you are fully aware. Evacuate first, then call for help. Plan ahead by keeping a phone, keys, shoes, and a warm layer easy to grab.

"I live in an apartment building"

Evacuate your unit, then notify building staff immediately. CO can originate in shared spaces, boiler rooms, garages, stacked vents, and affect neighboring units. Do not assume it is limited to your apartment. For apartment-specific guidance, see CO leak in an apartment.

Could It Be a False Alarm?

CO alarms can activate due to genuine CO, a failing sensor, low battery, or end-of-life behavior. Some environments (high humidity, dust, aerosols) can also cause issues on certain models. The safe sequence is always the same:

1. Evacuate and call for help if you cannot confidently rule out danger.
2. After clearance, troubleshoot the device, battery, reset, location, age.

If your unit is older, replacement is often the correct fix. Confirm proper placement using placement height guidance and the how many detectors checklist.

Sources & References

• CPSC: Carbon Monoxide Information Center
• NFPA: Carbon Monoxide Safety
• CDC: Carbon Monoxide Poisoning Prevention

Last updated: August 12, 2026