Carbon Monoxide Detector Going Off: What to Do First

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

Carbon Monoxide Detector Going Off: What to Do First

Short answer

Do not look for the source. The U.S. Consumer Product Safety Commission says to get everyone outside into fresh air at once, call 911 or the fire department, count heads, and not go back in until the responders say so. If a furnace, water heater or other appliance is the cause, it stays off until trained people have serviced it.

A carbon monoxide alarm going off is not something to troubleshoot first. Carbon monoxide is a colorless, odorless, poisonous gas, and the U.S. Consumer Product Safety Commission says it comes from fuel that does not burn completely, including natural gas, propane, oil and wood. A furnace, a water heater or a room heater that is not working properly can make it.

Everything on this page comes from the CPSC’s own carbon monoxide guidance. Read the next section before anything else.

Before you start

What to carry: only your phone, and a flashlight if it is dark. Getting everyone out in the first minutes matters more than anything else.

Gas or electric? The CPSC lists fuel burning appliances such as furnaces, ranges, water heaters and room heaters, and engines such as portable generators. An all electric furnace or heat pump burns no fuel.

Symptoms matter as much as the alarm. The CPSC says early symptoms are like the flu without a fever: headache, tiredness, shortness of breath, nausea and dizziness.

When the alarm sounds, the CPSC says: Do not try to find the source. Move outside to fresh air immediately.

Call for help from outside. Call 911, emergency services or the fire department.

Count heads, and stay out. Check everyone is accounted for. Do not go back in until the responders give you permission. The CPSC warns you could lose consciousness and die if you go back in.

What to do, in order

The CPSC's steps when a carbon monoxide alarm sounds. None of them is looking for the cause.

Check 1

Get out, call, stay out

Out. The CPSC says to move outside to fresh air immediately, and not to try to find the source of the carbon monoxide.

Call. Call 911, emergency services or the fire department. If anyone has symptoms, the CPSC says to see a doctor right away and say you suspect carbon monoxide poisoning.

Count. After calling 911, do a head count to make sure everyone is out.

Wait. Do not go back in until the emergency responders say you can.

After the responders leave

Check 2A service call

Keep the appliance off until it is serviced

If an appliance was the cause. The CPSC says not to operate it until it has been properly serviced by trained personnel.

If the alarm sounds again within 24 hours. The CPSC says to repeat the same steps, and have a qualified appliance technician check every fuel burning appliance for carbon monoxide and proper operation.

Anything not inspected. The CPSC says to note any combustion equipment the technician did not check, and ask its maker about carbon monoxide safety.

The garage. Make sure no car or engine is, or has been, running in an attached garage or next to the house.

How the furnace fits in

Check 3A yearly service

Have the heating inspected every year

Every year. The CPSC says to have the heating system professionally inspected and serviced every year, and the inspector should check the chimneys and flues for blockages, corrosion and loose or broken connections.

Leave the servicing to people who know it. The CPSC says never to service fuel burning appliances without the right knowledge, skill and tools, and to use the owner’s manual for minor adjustments.

Vents during building work. The CPSC says to make sure appliance vents and chimneys are not blocked by tarps or debris during renovations.

One fault to know. A cracked heat exchanger is one way a furnace can let combustion gases out: a cracked heat exchanger.

The alarm itself

Where the CPSC says carbon monoxide alarms should go, and where they should not.

Check 4The price of an alarm

Put alarms in the right places

The standard. The CPSC says to use an alarm that meets the current UL 2034 safety standard.

Where. On every level of the home and outside sleeping areas, battery powered or with battery backup. Interconnected alarms are best, because when one sounds, they all sound.

Where not. The CPSC says not near heating vents, not where furniture or curtains can cover them, and not in kitchens or above fuel burning appliances.

Testing. The CPSC says the test button checks the circuitry, not the accuracy of the sensor. Alarms also have a replacement age, given in their literature.

Things that make carbon monoxide

Check 5

Avoid the known causes

Generators and engines. The CPSC says never to run a portable generator or other engine in or near a garage, house or building, even with doors and windows open: heating and cooling after a power cut.

Heating with the wrong things. Never use a range, an oven or a clothes dryer to heat the home, and never run an unvented fuel burning appliance in a room where people sleep.

Ovens. Do not line the bottom of a gas or propane oven with foil, which the CPSC says blocks the combustion air and can produce carbon monoxide.

Cars. Never leave a car running in an attached garage, even with the garage door open.

Carbon monoxide levels

Level in the air	What the CPSC says
About 1 to 70 ppm	Most people feel no symptoms, but some heart patients may have more chest pain
Above 70 ppm and staying there	Symptoms become noticeable: headache, tiredness and nausea
Above 150 to 200 ppm, sustained	Disorientation, unconsciousness and death are possible

Questions people ask

What should I do if my carbon monoxide detector goes off?

Get everyone outside to fresh air, call 911 or the fire department, count heads, and do not go back in until the responders say so. Do not look for the source.

Can my furnace cause carbon monoxide?

Yes. The CPSC lists malfunctioning fuel burning appliances, including furnaces, water heaters and room heaters, among the causes of carbon monoxide deaths.

What are the symptoms of carbon monoxide poisoning?

At first, like the flu without a fever: headache, tiredness, shortness of breath, nausea and dizziness. Higher levels bring confusion, vomiting, loss of coordination and loss of consciousness.

The alarm went off again after the fire department left. What now?

Repeat the same steps. The CPSC says that if it sounds again within 24 hours, have a qualified appliance technician check every fuel burning appliance.

Where should carbon monoxide alarms go?

On every level and outside sleeping areas, with battery power or backup. Not near heating vents, not in kitchens, and not above fuel burning appliances.

Does pressing the test button prove the alarm works?

Only the circuitry. The CPSC says the test button does not check the sensor’s accuracy, and alarms have a replacement age.

Sources: U.S. Consumer Product Safety Commission (CPSC) "Carbon Monoxide Questions and Answers", Carbon Monoxide Information Center