Carbon Monoxide Detector Going Off: What to Do Now
If the alarm is sounding its full alarm pattern (not a single chirp):
Part of our Home Inspection Tools guide — start there for the full picture →
- Get everyone out now, including pets, into fresh air. Do not stop to search for the source or open windows.
- Call 911 or your gas utility’s emergency line from outside the house.
- Do not go back in until responders have checked the air and told you it is safe.
- Anyone with a headache, dizziness, nausea, confusion or drowsiness needs medical evaluation, even if they feel better outdoors.
A carbon monoxide detector going off is one of the few household alarms where the safest response never changes: leave first, sort it out later. Carbon monoxide (CO) has no color, smell or taste, so the alarm is the only warning most people get. Not every sound from a CO alarm is an emergency, though. A single chirp usually means a battery or end-of-life signal. This guide explains how to tell the patterns apart, the most common sources in Front Range homes, what responders check, and where alarms belong. It summarizes CPSC, CDC and EPA guidance and is not medical advice.
What It Means When a Carbon Monoxide Detector Is Going Off
A CO alarm sounds when its sensor has measured carbon monoxide above a set concentration for a set length of time. Most residential alarms sold in the U.S. are listed to the UL 2034 standard, which is designed so that alarms respond to sustained CO levels rather than brief, low spikes. In practice, that means a full alarm is the device telling you CO has been building up, not a momentary blip. Treat it as real until someone with a calibrated meter proves otherwise.
The CDC describes CO as a gas that “kills without warning” and notes that people who are sleeping, drunk, or under the influence of other substances can die from CO poisoning before they have symptoms. That is the core reason the response comes before the diagnosis. The CPSC’s advice is the same sequence printed in the box above: if you suspect CO poisoning, get outside to fresh air immediately, and then call 911.
The symptoms to watch for, according to the CDC, are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, often described as “flu-like.” The EPA adds a pattern worth remembering: CO can cause flu-like symptoms that clear up after leaving home. If several people or pets feel ill at the same time and improve outside, take that seriously even if no alarm has sounded.
Who is most at risk
The CDC says everyone is at risk for CO poisoning, and that infants, the elderly, and people with chronic heart disease, anemia or breathing problems are more likely to get sick from it. The EPA adds that effects vary greatly from person to person depending on age, overall health, and the concentration and length of exposure. That is why a household where one adult feels fine can still have a child or grandparent in trouble. Get everyone out together and let medical professionals decide who needs care.
What to tell the dispatcher
- That a CO alarm is sounding and everyone is out of the house.
- Whether anyone has symptoms, and who: age matters to responders.
- Which fuel-burning appliances the home has, and what was running when the alarm went off.
- Whether a car, generator or other engine was running in or near an attached garage.
Beep Patterns: Alarm, Low Battery, End of Life and Error
Most of the confusion around CO alarms comes from the different sounds they make. The patterns below are common across major brands, but they vary by manufacturer and model. The manual, or the label on the back of the alarm, governs. If you cannot tell what a sound means, treat it as an alarm and get outside.
| Sound | What it commonly means | What to do |
|---|---|---|
| Four quick beeps, a pause, repeating (often with a flashing red light) | CO alarm. The sensor has detected a dangerous level. | Get everyone out and call 911 or the gas utility from outside. |
| One short chirp roughly once a minute | Low battery on many models | Replace the battery. If chirping continues, check the end-of-life signal. |
| Several chirps (often five) about once a minute, or a display reading “End” or “ERR” | End of life on many models. The sensor has reached its rated service life. | Replace the whole alarm. A new battery will not fix it. |
| Irregular chirps, a fault light, or an error code | Malfunction or sensor error | Check the manual. Press test; if the fault returns, replace the unit. |
| Three beeps, or a long continuous tone, on a combination unit | Often the smoke alarm pattern on combination smoke and CO alarms | Treat as a fire alarm: get out and call 911. |
Combination units are where people get caught out. Many combination smoke and CO alarms use a three-beep pattern for smoke and four beeps for CO, and some also announce the hazard by voice. Our guides to the carbon monoxide detector beeping 3 times and the broader carbon monoxide detector beeping patterns go model by model. If the sound is a chirp you are trying to stop, start with why a carbon monoxide detector keeps beeping and our notes on the carbon monoxide detector battery.
Common Sources of Carbon Monoxide in a Home
CO forms whenever fuel burns incompletely. The EPA’s list of indoor sources includes leaking chimneys and furnaces; back-drafting from furnaces, gas water heaters, wood stoves and fireplaces; gas stoves; generators and other gasoline-powered equipment; automobile exhaust from attached garages; and worn or poorly adjusted combustion devices, especially when the flue is improperly sized, blocked, disconnected or leaking. In a typical Front Range house, the checklist looks like this.
Furnace and cracked heat exchanger
Gas forced-air furnaces run for months each winter along the Front Range. The heat exchanger separates combustion gases from the air blown through the house; if it cracks or rusts through, or if the flue is blocked or disconnected, combustion products can reach living space. A furnace that rumbles, rolls flames out of the burner area, leaves soot around the cabinet, or triggers the alarm soon after the heat kicks on is the first suspect. See our guides to a cracked heat exchanger and a professional furnace inspection.
Water heater backdrafting
Atmospheric gas water heaters rely on a draft hood and vent pipe to carry exhaust up and out. A disconnected or poorly sloped vent, a blocked chimney, or strong exhaust fans pulling air the wrong way can push exhaust back into the room. The CDC notes that horizontal vent pipes for appliances such as water heaters should go up slightly as they head outdoors, which helps prevent CO from leaking at joints. Our guide to the water heater vent pipe shows what proper slope and connections look like.
Gas range and oven
Gas burners produce some CO in normal use. The EPA reports that average CO levels in homes without gas stoves vary from 0.5 to 5 parts per million, levels near properly adjusted gas stoves are often 5 to 15 ppm, and levels near poorly adjusted stoves may be 30 ppm or higher. Using the oven to heat the house, running several burners for a long time without the range hood, or burners with yellow, lazy flames can all raise levels. The CDC is explicit: never heat your house with a gas oven.
Fireplace, wood stove and chimney
A closed damper, a chimney blocked by debris or a bird nest, or a gas log set that is out of adjustment can all spill CO into the room. The CDC recommends having your chimney checked or cleaned every year. Our chimney inspection guide covers what a sweep or inspector looks at.
Car in an attached garage
Warming up a car in an attached garage on a cold morning is a classic cause, even with the big door open. The CDC advises never running a car or truck inside a garage attached to a house, even with the garage door open. Exhaust can move into the house through the connecting door, attic spaces, and gaps around ducts.
Portable generator
Generators are among the deadliest sources. The CPSC notes that one portable generator can produce the same amount of carbon monoxide as hundreds of cars, and it advises using generators outside only, at least 20 feet from the home with exhaust facing away. Never run one in a garage, basement or crawl space, even with doors and windows open.
Other sources
Unvented kerosene or gas space heaters, charcoal grills used indoors or in a garage, and gas dryers with crushed or blocked vents round out the list. A gas smell is a different hazard from CO; if you smell rotten eggs, follow our gas leak smell guide and call the utility from outside.
What Responders Check When They Arrive
When the fire department or gas utility responds to a CO call, they bring calibrated meters that read the actual concentration in parts per million, something most household alarms do not display. A typical response includes:
- Air readings room by room, often starting at the door and working toward fuel-burning appliances, to find where levels are highest.
- Appliance checks at the furnace, water heater, boiler, range and fireplace, sometimes testing flue gases directly.
- Venting and draft checks, looking for disconnected or blocked vent pipes and backdrafting at draft hoods.
- The garage, including whether a vehicle or small engine has been run recently.
- Shutting off or red-tagging an appliance that is producing CO. A red tag means the appliance must not be used until a qualified technician repairs it.
Low readings when responders arrive do not prove the alarm was false. Leaving the doors open on the way out ventilates the house, and an appliance may have cycled off. Responders may ask you to run the furnace or water heater while they watch. Tell them what was running when the alarm sounded and whether anyone has symptoms.
False Alarm, Old Unit or Real Event?
A genuine false alarm is less common than people assume, but it happens. These are the usual explanations once responders find no CO source.
- End-of-life or failing sensor. CO sensors wear out. The CDC advises replacing CO detectors following the manufacturer’s instructions or every five years; many current alarms are rated for a longer service life, so check the date on the back and the manual. An expired unit can chirp, show an error, or behave erratically.
- Poor location. Alarms installed right beside a furnace, water heater, gas range, or the door to an attached garage can pick up brief spikes during startup. Manufacturer instructions usually specify minimum distances from fuel-burning appliances.
- Chemical interference. Some aerosol sprays, cleaning solvents, and high humidity near bathrooms can affect some sensors. The manual usually lists substances to keep away.
- Misread pattern. A low-battery chirp or a smoke alarm on a combination unit gets mistaken for a CO alarm.
An alarm that sounds repeatedly at the same time of day, or when the furnace starts, is not a false alarm until a technician has inspected that appliance. Intermittent CO from a marginal heat exchanger or a vent that only backdrafts in certain wind conditions is exactly the kind of problem that looks like a nuisance.
Resetting and silencing safely
Most alarms have a combined test and silence button. Pressing it during an alarm usually quiets the horn for a few minutes, but if CO is still present the alarm will sound again. Never silence a full alarm and stay inside. Once the home has been cleared and ventilated, many units reset on their own once CO levels drop; some require a button press or briefly removing the battery. If an alarm keeps going off after a reset and responders find nothing, replace it. Our guide to carbon monoxide detector replacement covers choosing a new unit, and the CO and gas detector shop lists current options.
Where CO Alarms Go and How Many a Home Needs
The CPSC recommends installing battery-operated CO alarms, or CO alarms with battery backup, on every level of the home and outside sleeping areas, and notes that interconnected alarms are best because when one sounds, they all sound. The CDC similarly recommends battery-operated or battery back-up CO detectors near every sleeping area.
In practice, a two-story house with a finished basement and bedrooms on the upper floor usually needs at least one alarm per level plus one in the hallway outside the bedrooms. Houses with bedrooms on more than one floor, or bedrooms far apart, need more. Common placement points:
- In the hall outside each separate sleeping area, close enough that the alarm will wake sleepers.
- On every level, including the basement where the furnace and water heater usually are, but not right next to them.
- Near the door from an attached garage, set back from it as the manual directs.
- Away from dead-air corners, bathrooms, and direct sunlight, and wherever the manufacturer’s instructions specify.
Consider a model with a digital readout. The CDC notes that this type can show the highest level of CO recorded in the home in addition to sounding an alarm, which is useful information for responders and technicians. For a room-by-room count, see how many carbon monoxide detectors you need, our carbon monoxide detector installation walkthrough, and our comparison of smoke and carbon monoxide detector combination units.
Colorado CO Alarm Requirements
Colorado has a state law requiring carbon monoxide alarms in many homes. As it is generally summarized, the law applies to dwellings that have a fuel-fired heater or appliance, a fireplace, or an attached garage, and it requires a working CO alarm near (commonly described as within about 15 feet of) each room used for sleeping. The requirement is triggered by events such as the sale or transfer of a home, new construction, and certain remodels or additions that need a permit. Rental units are generally covered as well.
Details, exceptions, and how the rule is enforced can vary, and local building departments in Denver, Douglas, El Paso, Jefferson and other Front Range counties may layer their own requirements on top. Buyers and sellers should confirm current requirements with their real estate agent, title company or local building department rather than relying on a summary. Home inspectors along the Front Range commonly note missing or expired CO alarms in their reports, so it pays to check dates and placement before listing.
After the All-Clear: Preventing the Next Alarm
Once responders have cleared the house, the work shifts to finding and fixing the cause so the alarm does not go off again. The EPA’s first recommendation is to be sure combustion equipment is maintained and properly adjusted, and to have a trained professional inspect, clean and tune up the central heating system, including furnaces, flues and chimneys, every year. The CDC recommends annual servicing of the heating system, water heater, and any other gas, oil or coal burning appliances by a qualified technician.
- Do not relight or restart an appliance that was shut off or tagged until a licensed technician has repaired it.
- Schedule service before heating season. Early fall appointments catch heat exchanger and venting problems before the first cold snap.
- Check batteries twice a year. The CDC suggests checking CO detector batteries when you change the clocks each spring and fall.
- Write the install date on each alarm and set a calendar reminder for its replacement date.
- Plan generator placement now, before a winter storm knocks the power out, at least 20 feet from the house, per CPSC guidance.
- Keep the garage door rule simple: no idling cars, mowers, or snow blowers in an attached garage.
Many buyers also ask for a CO check as part of a home inspection. Inspectors look at venting, flue connections, signs of backdrafting and alarm placement; the broader home inspection tools guide and our hub for inspection tools explain the instruments used, including combustion gas analyzers and handheld CO meters.
The CDC estimates that each year more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 100,000 visit an emergency department, and more than 14,000 are hospitalized. A working alarm in the right place, an annual appliance check and a rule of leaving first are what keep a household out of those numbers.
References
- Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission
- Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention
- Carbon Monoxide Poisoning: Fact Sheets and Other Resources — Centers for Disease Control and Prevention
- Carbon Monoxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency
Frequently asked questions
What should I do if my carbon monoxide detector goes off but nobody feels sick?
Leave anyway. Get everyone and any pets outside, then call 911 or your gas utility from outside. CO has no smell, and the CDC notes that sleeping people can be overcome before they have symptoms, so feeling fine is not proof the air is safe.
How can I tell a CO alarm from a low-battery chirp?
On many models, a CO alarm is four quick beeps and a pause, repeating loudly, while a low battery is a single short chirp about once a minute. End-of-life signals are often several chirps per minute. Patterns vary by brand, so check the label on the back of the alarm or the manual.
Can a carbon monoxide detector go off for no reason?
Occasionally, usually because the sensor has reached the end of its life, the alarm sits too close to a fuel-burning appliance, or chemicals nearby are affecting the sensor. Treat every full alarm as real until responders with a calibrated meter have checked the house and your appliances.
Will the fire department come out for a CO alarm?
Most fire departments respond to CO alarm calls, and many gas utilities will also send a technician. They use meters that read actual CO levels, check fuel-burning appliances and venting, and can shut off an appliance that is producing CO.
Does Colorado require carbon monoxide detectors when selling a house?
Generally, yes. Colorado law requires CO alarms near sleeping rooms in many homes with fuel-burning appliances, fireplaces or attached garages, and the sale or transfer of a home is one of the triggers. Confirm current details with your agent or local building department.
Buying a home with an older furnace or water heater along the Front Range? Send us a note and we can connect you with a local inspector who checks venting, combustion appliances and CO alarm placement.
CO & gas detectors
A carbon monoxide alarm is the cheapest life-safety device in the house. Combination CO + explosive-gas units add a layer near furnaces and gas lines.
| Product | Why | Buy |
|---|---|---|
 First Alert CO Alarm | Plug-in with battery backup. | Amazon — $21.99 |
 Kidde Nighthawk CO + Gas | Detects CO + natural/propane gas. | Amazon — $57.47 |