Carbon Monoxide Alarms: Where to Put Them, How to Test Them and Why They Fail When You Need Them

Quick answer: install carbon monoxide (CO) alarms outside each sleeping area and on every level of your home, following the manufacturer’s mounting instructions and any local code, as the U.S. Consumer Product Safety Commission (CPSC) and the National Fire Protection Association (NFPA) recommend. Choose alarms listed to the UL 2034 standard, test them monthly with the test button and replace them on the manufacturer’s schedule, since sensors wear out. Make sure at least one alarm has battery backup for power outages, when generators and fuel-burning heaters raise the risk. If an alarm sounds, get everyone outside to fresh air at once and call 911 from outside.

Carbon monoxide is a gas you cannot see, smell or taste. It comes from burning fuel: gas furnaces and water heaters, stoves, fireplaces, cars and, most dangerously during storms, portable generators and improvised heating. The CDC describes the symptoms of CO poisoning as often “flu-like,” including headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. People who are asleep can die before they notice anything is wrong.

A working alarm is the only way most households will know CO is building up. Severe weather makes this more urgent, because power outages lead people to run generators, use camp stoves or start cars in garages. This guide explains where alarms go, how to keep them working and why they sometimes fail. It is part of our severe weather preparedness guide.

Sources: U.S. Consumer Product Safety Commission, carbon monoxide safety guidance and generator safety resources; National Fire Protection Association, carbon monoxide safety tips and NFPA 72 and NFPA 2 overviews; U.S. Fire Administration, carbon monoxide alarm guidance; Centers for Disease Control and Prevention, carbon monoxide poisoning symptoms and prevention; UL Standards and Engagement, UL 2034 standard for single and multiple station CO alarms. Check your state and local codes and your alarm’s manual. Checked October 2026.

Why CO Risk Rises During Storms and Outages

Most of the year, CO risk comes from fuel-burning appliances that are not working properly or from blocked vents. During and after severe weather, new sources appear quickly:

• Portable generators run too close to the home, in a garage or near open windows
• Gas ovens or stoves used for heat when the furnace is out
• Charcoal grills and camp stoves used indoors or in enclosed porches
• Cars left running in attached garages to charge phones or stay warm
• Snow-blocked vents on furnaces, water heaters and dryers after winter storms

The CPSC repeatedly warns that generators should never be used inside a home, garage, basement, crawlspace or shed, even with doors and windows open, and should be placed outdoors far from windows, doors and vents. Our guide to generator safety, carbon monoxide and fuel storage covers that side in detail.

Because outages are exactly when these risks spike, your alarms need to keep working without wall power.

Where to Install Carbon Monoxide Alarms

The NFPA and CPSC give the same core placement advice:

1. Outside each separate sleeping area, in the hallway near bedrooms, so the alarm can wake people.
2. On every level of the home, including the basement.
3. In other locations required by law or code in your area.
4. According to the manufacturer’s instructions for height and position.

Mounting height surprises many people. CO mixes with air, so there is no universal “high” or “low” rule. Some alarms are designed for the ceiling, some for the wall and plug-in models sit near outlet level. Follow the instructions that came with your model.

Places to avoid

Manufacturers commonly list locations that cause false alarms or slow detection. Typical examples include:

• Directly next to fuel-burning appliances, where brief bursts at startup may trigger nuisance alarms
• Bathrooms and other humid areas
• Near open windows, fans or air vents that dilute the air reaching the sensor
• Behind curtains or furniture, or in dead-air spaces such as wall corners
• Locations in direct sunlight or extreme temperatures

Distances vary by model, so check your manual rather than relying on a general rule.

Apartments and condos

In multi-unit buildings, CO can come from your own appliances, a neighbor’s unit, a shared boiler room or an attached parking garage. Many states and cities require landlords to install CO alarms in rental units, though the details vary. If your unit has fuel-burning appliances, an attached garage or shares walls with a boiler room, ask your landlord which alarms are installed and where.

Renters can usually add a plug-in or battery CO alarm without permission because it does not require drilling or wiring. Never remove or disconnect an alarm your landlord installed.

Choosing the Right Type of Alarm

All CO alarms do the same basic job, but power options and features differ. That matters most during outages.

Type	How it is powered	Outage performance	Good fit
Battery-only	Replaceable or sealed long-life battery	Keeps working through outages	Renters, quick installs, bedrooms
Plug-in with battery backup	Wall outlet plus backup battery	Works during outages if backup is good	Hallways with a convenient outlet
Hardwired with battery backup	Home electrical system plus backup	Works during outages if backup is good	New construction, many code requirements
Combination smoke and CO	Any of the above	Same as its power source	Saving ceiling space, interconnected systems

Look for alarms listed to UL 2034, the safety standard for residential CO alarms. A plug-in alarm with no battery backup will go silent in a power outage, exactly when generator and heater risks are highest.

Interconnected alarms, where one sounding sets off all the others, are especially useful in larger homes. The NFPA recommends interconnection for smoke alarms so everyone hears a warning anywhere in the home, and many CO alarm systems can be connected the same way.

How to Test and Maintain CO Alarms

An alarm only protects you if it works. The U.S. Fire Administration and NFPA recommend simple, regular maintenance:

• Test monthly by pressing the test button. This confirms the horn and circuitry work.
• Replace batteries on the schedule the manufacturer recommends. Many households pair this with daylight saving time changes as a reminder.
• Keep alarms clean by gently vacuuming or dusting the vents as the manual describes.
• Replace the whole alarm when it reaches the end of its rated life. Sensors degrade over time, and most alarms have a replacement date printed on the back.
• Never paint over an alarm or cover it with decorations.

Write the install date on the back of each new alarm with a marker. That small step answers the “how old is this?” question years later.

Why CO Alarms Fail When You Need Them

Most failures are predictable, which means most are preventable.

Dead or removed batteries. Battery-powered and backup-battery alarms stop working when batteries are dead. People sometimes pull batteries to stop a low-battery chirp and forget to replace them.

Expired sensors. Every CO sensor has a limited life. When an alarm reaches end of life, many models emit a distinct chirp pattern or display an end-of-life message. That signal means replace the entire unit, not just the battery.

Wrong location. An alarm in a basement storage room may not wake anyone upstairs. Alarms behind doors or in dead air spaces may detect gas late.

No backup power. Plug-in alarms without batteries go silent in outages.

Silenced and ignored. Some alarms have a hush button for nuisance alarms. Silencing an alarm without finding the cause defeats its purpose.

Too few alarms. A single alarm on one floor of a multi-level home may not be heard behind closed bedroom doors.

Symptom	Likely meaning	What to do
Single chirp every minute or so	Low battery (on many models)	Replace battery right away
Different repeating chirp pattern or end-of-life message	Sensor end of life	Replace the whole alarm
Full alarm pattern (often four beeps and a pause)	CO detected	Get out to fresh air, call 911
No sound when testing	Dead battery or failed unit	Replace battery; if still silent, replace alarm

Chirp patterns differ between brands, so keep the manual or a photo of its troubleshooting page with your emergency documents.

What to Do When a CO Alarm Sounds

Treat every CO alarm as real until emergency responders say otherwise. The NFPA and CPSC advise:

1. Move everyone to fresh air immediately, outside or to an open door or window, including pets.
2. Make sure everyone is accounted for. Do not go back inside for belongings.
3. Call 911 or the fire department from outside or from a neighbor’s home.
4. Stay out until responders say it is safe to re-enter.
5. Get medical help for anyone with symptoms such as headache, dizziness or nausea.

Do not assume an alarm is a false alarm because nobody feels sick. Symptoms may appear later, and people who are asleep or intoxicated may not notice them at all.

If the alarm went off during a power outage, look at what was running: a generator near a window, a grill in the garage or a stove used for heat. Fix the source before going back to normal routines.

Preventing CO Buildup in the First Place

Alarms warn you about a problem; prevention keeps the problem from starting. The CPSC recommends having heating systems, water heaters and other fuel-burning appliances inspected by a trained professional every year, ideally before the heating season. Chimneys and flues should be checked for blockages, corrosion and loose connections.

Everyday habits that cut CO risk:

• Never run a car in an attached garage, even with the garage door open.
• Use exhaust fans when cooking on a gas range, and never use the oven to heat the home.
• Keep fuel-burning space heaters vented as the manufacturer requires, and choose models with safety shutoffs if you use them at all.
• Check outside vent openings for snow, leaves, nests and debris, especially after storms.
• Watch for warning signs at appliances, such as soot, a yellow or flickering burner flame instead of a steady blue one, or excess moisture on windows near a furnace.

High-rise and apartment residents should also report odd odors, stuffy air in hallways or recurring alarm activations to building management right away, since the source may be in a shared system. If your building loses power, our guide on preparing for a power outage in a high-rise covers safe lighting and heating alternatives.

A CO Safety Checklist for Storm Season

Before severe weather season, take a few minutes to check every alarm and plan for outages:

• Test every CO alarm and replace any past its end-of-life date.
• Install fresh backup batteries in plug-in and hardwired units.
• Confirm there is an alarm outside each sleeping area and on every level.
• Plan generator placement outdoors and away from openings before you ever need it.
• Keep vents clear of snow, leaves and debris after storms.
• Never use grills, camp stoves or gas ovens for heat indoors.
• Review the plan with everyone at home, including what the alarm sounds like.

Our guides to winter storm preparedness for your home and car and an apartment fire safety plan pair well with this checklist.

Frequently Asked Questions

Where should a carbon monoxide alarm be placed?

Outside each sleeping area and on every level of the home, according to CPSC and NFPA guidance, plus anywhere your local code requires. Follow the manufacturer’s instructions for mounting height.

How often should I test a CO alarm?

Monthly, using the test button. Replace batteries on the manufacturer’s schedule and replace the whole alarm at the end of its rated life.

Do CO alarms work during a power outage?

Battery-only alarms and plug-in or hardwired alarms with working backup batteries continue to work. Plug-in alarms without a battery do not.

Can a smoke alarm detect carbon monoxide?

Standard smoke alarms do not detect CO. Combination smoke and CO alarms do both; check the label.

What should I do if my CO alarm goes off?

Get everyone to fresh air immediately, call 911 from outside and do not go back in until responders say it is safe.

The Bottom Line

Carbon monoxide alarms are cheap compared with what they protect, but only if they are in the right places and actually working. Put one outside every sleeping area and on every level, choose UL 2034 alarms with battery backup, test monthly and replace units at end of life. During storms and outages, when generators and improvised heating raise CO risk, those alarms may be the only warning you get.