Safety first: official instruction
If the CO alarm is sounding right now
The U.S. Consumer Product Safety Commission’s instruction, quoted: “If the alarm signal sounds do not try to find the source of the CO:”
- “Immediately move outside to fresh air.”
- “Call your emergency services, fire department, or 911.”
- “After calling 911, do a head count to check that all persons are accounted for. DO NOT reenter the premises until the emergency services responders have given you permission.”
CPSC adds that if responders let you back in “and your alarm reactivates within a 24 hour period, repeat steps 1, 2 and 3 and call a qualified appliance technician.”
Source: U.S. Consumer Product Safety Commission: Carbon Monoxide Fact Sheet. NFPA gives the same order: get to fresh air, account for everyone, and call for help from there[2].
Symptom guide
CO detector going off? What the sounds mean and what to do
By the HeatAndAirCost editors · Last updated October 5, 2026 · How we research prices
What people commonly report
People describe three different sounds: a loud, repeating alarm pattern; a single short chirp every minute or so; and a chirp or screen message from an alarm that has been on the wall for years. NFPA and CPSC handle these very differently. The first is the alarm signal and gets the response in the red box above. The other two are usually a trouble or end-of-life signal. If you cannot tell which one you are hearing, treat it as the alarm.
Why official sources treat it as an emergency
Carbon monoxide is, in CPSC’s words, “a deadly, colorless, odorless, poisonous gas.” Because you cannot see or smell it, the alarm is often the only warning[1]. CPSC counts more than 200 U.S. deaths a year from CO produced by non-automotive consumer products, and lists malfunctioning furnaces, ranges, water heaters and room heaters among them, along with portable generators, fireplaces and charcoal burned indoors[1].
The early symptoms CPSC lists are headache, fatigue, shortness of breath, nausea and dizziness, which it notes are similar to the flu but without a fever. EPA adds that CO can cause flu-like symptoms that clear up after leaving home[3]. If anyone in the house has these, tell the 911 dispatcher.
What official sources say can produce CO in a home
These are the sources the agencies name. They are possibilities for responders and technicians to check, not a diagnosis of your home.
- Fuel-burning appliances that are malfunctioning. Furnaces, ranges, water heaters and room heaters are on CPSC’s list[1]. EPA specifically names back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, and worn or poorly adjusted combustion equipment[3].
- Engines running in or next to the house. Portable generators top CPSC’s product list. NFPA warns that a vehicle or generator running in an attached garage can produce dangerous CO levels, even with the garage door open[2].
- Blocked vents. NFPA says to keep dryer, furnace, stove and fireplace vents clear of snow during and after a storm[2]. Trane notes that a furnace whose flue or vent pipe is blocked will shut itself down to prevent carbon monoxide poisoning, which can show up as a furnace that keeps cutting out[5].
- A burner or heat exchanger problem. ENERGY STAR’s maintenance checklist says a dirty burner or cracked heat exchanger causes improper burner operation, and that faulty gas connections are a fire hazard and can contribute to health problems[4].
How to find out which signal it is, and what happens next
- Alarm pattern sounding: follow the CPSC steps above. Finding the source is the job of the fire department, your gas utility, or a qualified appliance technician, not yours[1].
- Single chirp: NFPA’s guidance for the audible trouble signal is to check for a low battery and replace it. “If it still sounds, call the fire department”[2].
- Old alarm chirping or showing a message: CPSC says alarms have a recommended replacement age, found in the product literature or from the manufacturer[1]. The alarm’s own manual or label explains which sound pattern means what.
- After responders clear the house: CPSC says to have the heating system professionally inspected and serviced every year. The U.S. Fire Administration gives the same once-a-year advice for heating equipment and chimneys[6]. ENERGY STAR’s checklist covers gas connections, gas pressure, burner combustion and the heat exchanger[4].
Affiliate links
Replacing an expired alarm or adding coverage
Only for after the home has been cleared, or when an alarm has reached its replacement age. CPSC says to choose an alarm that meets the current UL 2034 standard and to place one on every level and outside sleeping areas. An alarm warns you; it does not fix whatever produced the CO.
As an Amazon Associate we earn from qualifying purchases. It does not change your price. These are screening or upkeep items. None of them is a repair, and none replaces a licensed technician.
When to stop and call
- The alarm signal is sounding: leave and call 911 from outside, every time.
- It goes off again within 24 hours of being cleared: repeat the steps and get a qualified appliance technician to check every fuel-burning appliance, per CPSC[1].
- The trouble chirp continues after a fresh battery: NFPA says call the fire department[2].
- You also smell gas: that is a gas-leak response. See furnace smells like gas.
Related guides
What the repair can cost
Related problems
Sources
This page is general information collected from the official and manufacturer sources listed above. It is not an inspection or a diagnosis of your system, and only a qualified technician who sees the equipment can tell you what is wrong. For a sounding CO alarm, the instruction is CPSC’s: get outside and call 911. HeatAndAirCost is not affiliated with the U.S. Consumer Product Safety Commission, NFPA, the EPA, ENERGY STAR, the U.S. Fire Administration, Trane. Found an error? hello@heatandaircost.com.
Getting local help
Finding a qualified technician
We do not inspect systems, rate contractors, or recommend any particular company. Whoever you call, ask for the license number and look it up on your state or county licensing board’s public site before work starts. Refrigerant work also requires an EPA Section 608 certified technician. Our contractor hiring checklist covers licenses, insurance, written quotes and red flags.
Advertising partner
Get quotes from local pros
Angi is a national marketplace for home services. You describe the job, and heating and furnace contractors who work in your area contact you with their own quotes. Requesting quotes is free and you are never obliged to hire anyone.
Angi pays us a referral fee for a completed service request. The fee comes from Angi, not from you, and it does not change the cost figures on this page.
People also ask
CO detector going off: FAQs
What should I do if my carbon monoxide detector goes off?
The U.S. Consumer Product Safety Commission says not to look for the source. Move outside to fresh air right away, call 911 or the fire department, count heads to make sure everyone is out, and do not go back in until emergency responders say you can.
Why is my CO detector chirping instead of alarming?
NFPA calls this the audible trouble signal. Its guidance is to check for a low battery and replace it, and to call the fire department if the signal keeps sounding after that. CO alarms also have a replacement age, which CPSC says is listed in the product literature or available from the manufacturer.
Can a furnace set off a carbon monoxide alarm?
CPSC lists malfunctioning fuel-burning appliances, furnaces included, among the consumer products behind more than 200 U.S. CO deaths a year. EPA names back-drafting from furnaces and poorly adjusted combustion equipment as indoor CO sources. Only responders and a qualified technician can say what set off a particular alarm.
Where should carbon monoxide alarms be installed?
CPSC says to put battery-operated or battery-backup CO alarms on every level of the home and outside sleeping areas, away from heating vents and anywhere furniture or drapes could cover them. CPSC does not recommend installing them in kitchens or above fuel-burning appliances.
Can I go back inside once the CO alarm stops beeping?
Not on your own. CPSC says to stay out until emergency responders give permission. If they let you back in and the alarm sounds again within 24 hours, CPSC says to repeat the same steps and have a qualified appliance technician check every fuel-burning appliance.