Carbon monoxide alarm going off: what to do now

Updated October 26, 2026. General information only, not professional advice.

A carbon monoxide (CO) alarm is a warning to take seriously every time. CO is a gas you cannot see or smell, and it can kill. Treat the alarm as real until emergency responders tell you otherwise.

Do this right now

Get everyone outside to fresh air. Bring children and pets. Do not stop to look for the source or open every window.
Call 911 from outside. The CPSC says that if you suspect CO poisoning, get outside to fresh air immediately, and then call 911.
Count heads. Make sure everyone in the home is out.
Stay out. Do not go back inside, even briefly, until the fire department or another responder says it is safe.

If anyone has symptoms, tell the 911 dispatcher right away so medical help can come too.

Know the symptoms

The CDC lists the most common symptoms of CO poisoning as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. It also notes that symptoms are often described as flu-like.

Warning signs that point to CO rather than a bug:

Several people, or people and pets, feel sick at the same time.
Symptoms ease when you leave the house and return when you come back.
Symptoms started after the heat came on, a generator was running, or a car was warming up in an attached garage.

The CDC also warns that people who are sleeping or have been drinking alcohol can die from CO poisoning before they ever have symptoms. That is why a working alarm matters so much.

Common sources of CO in a home

CO comes from burning fuel. Common sources include:

Furnaces, boilers and water heaters with a blocked vent, cracked part or poor combustion.
Gas ranges and ovens, especially when used for heat. Ready.gov warns never to heat your home with a gas stovetop or oven.
Fireplaces and wood stoves with a blocked chimney.
Generators. Ready.gov says generators should always be used outdoors and at least 20 feet away from windows, doors and attached garages.
Cars, mowers and other engines running in an attached garage.
Grills, camp stoves and other outdoor equipment brought inside.

After responders say it is safe

Once the home has been cleared, the source still needs to be found and fixed, or the problem will come back.

Do not use any fuel-burning appliance that responders flagged until it has been inspected and repaired.
Have a qualified technician check your furnace or boiler, water heater, venting and chimney.
Ask what was found and get it in writing.

What if the alarm is chirping, not sounding?

A short chirp every so often usually means a low battery or an alarm that has reached the end of its life. This is different from the full alarm pattern. Check the label or manual for your model. If you are not sure which sound you are hearing, treat it as a real alarm, get outside and call 911.

Prevent it from happening again

Install CO alarms. The CPSC recommends battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas. Check the batteries regularly.
Get yearly service. Have your heating system, water heater and any other fuel-burning appliances checked by a qualified technician each year.
Keep vents clear. Make sure outside vent pipes are not blocked by snow, ice, leaves or nests.
Never use these indoors: generators, grills, camp stoves or a gas oven for heat.
Never run an engine in a garage, even with the door open.

When to call a pro

Call 911 first whenever the alarm sounds. After the home has been cleared, call an HVAC technician to inspect and repair the furnace, boiler, water heater or venting that may be the cause. If the alarm keeps going off, do not ignore it or remove its batteries.

Before you hire anyone, verify that they hold any license your state or city requires for HVAC or gas appliance work, and ask for proof of insurance. Get a written estimate before work starts, and ask the technician to explain what caused the problem.