The Best Carbon Monoxide Detectors

What UL 2034 actually requires

This is the single most useful thing to understand before buying, and almost nobody explains it. UL 2034 is the standard for single- and multiple-station carbon monoxide alarms, and it specifies response times paired with concentrations:

At 70 ppm, the alarm must sound within 60 to 240 minutes. At 150 ppm, within 10 to 50 minutes. At 400 ppm, within 4 to 15 minutes. And — critically — the alarm must not sound below 30 ppm at all.

That last requirement is deliberate. It exists to prevent nuisance alarms, because low concentrations of CO occur in ordinary life and an alarm that fired at 15 ppm would be silenced within a week. CPSC's conformance testing work and the NFPA research foundation's review of alarm thresholds both work through the reasoning.

So a listed CO alarm is designed to prevent acute poisoning in a healthy adult. It is not designed to tell you the air is slightly bad, and for most households that is the right engineering. For some households it is not, and low-level monitors is the page for that.

How we ranked these

We ran no tests. Every unit here is UL 2034 listed, which means it has already passed the response-time testing above — that is third-party evidence and it is more than we could generate.

So the scoring is about everything the listing does not decide. Listing and compliance is whether the unit is properly listed and how long the manufacturer states it lasts. Reading and display is whether it reports a number or just alarms. Peak-level memory is whether it records a concentration you were not present for, which is the difference between finding a slow leak and missing one. Power resilience is what happens in an outage — the hours when CO risk is highest. Placement freedom is whether the unit can go where the room needs it or only where an outlet is.

What is not in the list: price, which we do not display, and star ratings, which we will not launder into a score.

Quick picks

Scored against listing and compliance, reading and display, peak-level memory, power resilience and placement freedom. Tap a row for the full write-up. No price is shown anywhere on this site because we do not cache retailer prices.

#	Product	Best for	Score	Price
1	Kidde Nighthawk KN-COPP-3 Plug-In CO Alarm Peak-level memory down to 11 ppm is the feature that finds a slow leak.	Most homes with an outlet	8.4/ 10	Check price on Amazon #ad how we are funded · we do not cache prices
2	First Alert CO710 Sealed-Battery CO Alarm Sealed ten-year battery, so the position is free and there is nothing to maintain.	Positions with no outlet	8.2/ 10	Check price on Amazon #ad how we are funded · we do not cache prices
3	First Alert CO615 Plug-In CO Alarm with Battery Backup Outlet height puts the display where a human can see it without a stepladder.	A hallway you will actually read	7.8/ 10	Check price on Amazon #ad how we are funded · we do not cache prices
4	Kidde KN-COPP-B-LPM Battery CO Alarm with Digital Display Battery only with a display and peak memory — the portable option.	Travel, cabins and rooms with no power	8.1/ 10	Check price on Amazon #ad how we are funded · we do not cache prices

The four alarms in full

Illustrative photo

1. Most homes with an outlet

Kidde Nighthawk KN-COPP-3 Plug-In CO Alarm

Peak-level memory down to 11 ppm is the feature that finds a slow leak.

Listed to

UL 2034

Sensing / agent

Electrochemical CO sensor

Power / service life

10-year unit life; 9V battery backup

Listing and compliance9.0

Reading and display9.0

Peak-level memory9.5

Power resilience8.5

Placement freedom6.0

Overall8.4/ 10

The peak-level memory is why this is first, and it is worth explaining because it is the least-marketed useful feature in the category. A cracked heat exchanger or a partially blocked flue often produces a modest, intermittent leak — enough to matter over a winter, not enough to trip a UL 2034 alarm, which is forbidden from alarming below 30 ppm. A unit that records a peak down to 11 ppm gives you evidence of that. You come home, press the peak button, and see 22 ppm from some point during the day. That is a service call rather than a shrug.

The battery backup matters more than it sounds. Power outages are exactly when households run generators and fire up alternative heating, which is when CO risk peaks. A plug-in alarm with no backup is asleep during the most dangerous hours of the year.

On the display: the live readout begins at 30 ppm, which is the UL 2034 floor. Below that it shows nothing, which looks like a fault and is not. That is the standard's design, not the alarm's shortcoming — and it is exactly why low-level monitors exist as a separate product class.

Do not mount it directly above a furnace or water heater. Fuel-burning appliances produce a brief spike on start-up, and an alarm in the plume will nuisance-trip. Where to install CO detectors has the positions.

What it does well

• +UL 2034 listed, with a 10-year unit life and a 10-year limited warranty
• +Digital display reads 30 to 999 ppm and refreshes every 15 seconds
• +Peak-level memory records concentrations as low as 11 ppm — far below the alarm threshold
• +Direct-plug, 6 ft cord or tabletop mounting, with 9V battery backup for outages

The downside

• −Tied to an outlet position, so it cannot go wherever the room needs it
• −The live display starts at 30 ppm, so a low reading shows as zero until you check the peak memory
• −The 9V backup is a battery you have to remember, unlike a sealed unit

Do not buy this if: the position you need has no outlet, or you want a display that reads live down to single-digit ppm. For the first, buy the sealed-battery CO710 below; for the second you are looking for a low-level monitor, which is a different instrument entirely.

• Digital display reads 30 to 999 ppm and refreshes every 15 seconds
• Peak-level memory records concentrations as low as 11 ppm — the feature that catches a chronic leak
• Direct-plug, 6 ft cord or tabletop mounting
• Battery backup keeps it monitoring through a power cut, which is exactly when generators come out

Kidde product documentation

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Illustrative photo

2. Positions with no outlet

First Alert CO710 Sealed-Battery CO Alarm

Sealed ten-year battery, so the position is free and there is nothing to maintain.

Listed to

UL 2034

Sensing / agent

Electrochemical CO sensor

Power / service life

10-year sealed battery

Listing and compliance9.0

Reading and display8.0

Peak-level memory6.5

Power resilience8.0

Placement freedom9.5

Overall8.2/ 10

The argument for a sealed-battery unit is placement freedom, and in CO detection that matters more than it does for smoke alarms. Because carbon monoxide mixes with air rather than reliably rising, manufacturers' instructions vary on height — and a plug-in alarm has already made that decision for you by being at outlet height.

It also matters for hearing it. The requirement that actually saves lives is that the alarm wakes people, which means it needs to be where the bedrooms are rather than where the wiring is. A sealed unit goes on a bedroom wall or a landing ceiling without an extension lead.

The trade is that a sealed unit is a ten-year consumable. That is the correct design for a device whose electrochemical sensor has a finite life anyway — replacing the sensor and the horn and the housing together is cleaner than pretending a fresh battery restores a decade-old cell. Lifespan and replacement covers the arithmetic.

What it does well

• +10-year sealed battery — no outlet needed and nothing to replace for a decade
• +Digital display showing the measured concentration
• +Goes at whatever height and position the room actually needs
• +No low-battery chirp until the unit is genuinely at end of life

The downside

• −At end of life the whole unit is replaced, by design
• −No mains power, so nothing keeps it running if the sealed cell fails early
• −Peak-memory behavior is less generous than the Kidde's 11 ppm floor

Do not buy this if: you want the cheapest possible per-unit cost and you have outlets everywhere. A plug-in unit with a replaceable backup battery is less expensive up front; the sealed unit wins over ten years and wins on placement.

• Sealed 10-year battery — nothing to change, and no low-battery chirp until it is genuinely done
• Digital display showing the measured concentration
• Placement is free: no outlet required, so it can sit at the height the room actually needs
• End of life means replacing the whole unit, by design

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Illustrative photo

3. A hallway you will actually read

First Alert CO615 Plug-In CO Alarm with Battery Backup

Outlet height puts the display where a human can see it without a stepladder.

Listed to

UL 2034

Sensing / agent

Electrochemical CO sensor

Power / service life

AC powered with battery backup

Listing and compliance9.0

Reading and display8.5

Peak-level memory7.0

Power resilience8.5

Placement freedom6.0

Overall7.8/ 10

The underrated virtue of a plug-in CO alarm is that people actually look at it. A ceiling alarm is checked when it beeps; a wall unit at eye-ish height gets glanced at, which means an unusual reading gets noticed.

That makes this a good second or third unit in a house that already has combination alarms on the ceilings — somewhere on the route between the bedrooms and the stairs.

The failure mode is human. Plug-in devices in hallways get unplugged for vacuum cleaners and phone chargers. If you buy one, put a note on the socket, or pick the socket nobody uses.

What it does well

• +AC powered with battery backup, so an outage does not silence it
• +Digital display at outlet height — readable and testable without climbing
• +Nothing to install: it plugs in
• +A sensible second unit for a hallway where a ceiling alarm already covers smoke

The downside

• −Outlet-bound placement
• −A plug-in unit in a hallway is easy to unplug for a vacuum cleaner and forget
• −Backup battery is a maintenance item

Do not buy this if: the room has no convenient outlet, or the outlet you would use is behind furniture. An alarm plugged in behind a sofa is neither audible nor readable.

• Plugs in at outlet height, which is where a hallway alarm is easiest to read and test
• Battery backup keeps it alive through an outage
• Digital display of the current reading
• Tied to an outlet position, so it cannot go wherever you like

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Illustrative photo

4. Travel, cabins and rooms with no power

Kidde KN-COPP-B-LPM Battery CO Alarm with Digital Display

Battery only with a display and peak memory — the portable option.

Listed to

UL 2034

Sensing / agent

Electrochemical CO sensor

Power / service life

Battery powered; 10-year unit life

Listing and compliance9.0

Reading and display8.5

Peak-level memory9.0

Power resilience6.5

Placement freedom9.5

Overall8.1/ 10

The travel case for a CO alarm is real and under-appreciated. Rented cabins, older holiday lets and RVs are exactly where fuel-burning appliances are poorly maintained and where you have no idea what detection exists. A battery alarm with a display in a bag costs very little and has saved people.

It is also the right unit for a room in a house with no accessible outlet and no interest in ceiling work — a garage office, a basement corner, a workshop.

The replaceable battery is the honest downside. A sealed unit cannot be left flat; this one can, so it needs to be on the annual list. The home fire safety checklist has a place for it.

What it does well

• +Battery only, so it goes anywhere — a cabin, a rental, a hotel room, an RV
• +Digital display and peak-level memory, unusual in a battery unit
• +Genuinely portable, which more people should take advantage of
• +No outlet, no wiring, no compromise on where it sits

The downside

• −Replaceable batteries mean an actual annual maintenance task
• −No mains power, so a flat battery is a silent failure
• −Less convenient than a sealed unit for a permanent install

Do not buy this if: you want a fit-and-forget permanent alarm. For that the sealed-battery CO710 is the better answer — this one's advantage is that it travels.

• Battery only, so it goes where there is no outlet — a bedroom wall, a cabin, a rental
• Digital display plus peak-level memory
• Portable enough to travel with, which matters more than most buying guides admit
• Replaceable batteries mean an annual check is genuinely required

Kidde product documentation

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Where they go

One per level, and within earshot of every sleeping area. Carbon monoxide poisoning most often kills people who were asleep, so the position that matters is the one that wakes them.

One in or near the space with the furnace or water heater — but not mounted directly above the appliance, where the start-up plume will nuisance-trip it.

On the house side of a shared wall with an attached garage. A car started in an attached garage is one of the commonest residential CO events, and it does not require the car to be left running long.

High or low is the wrong question. CO has nearly the same density as air: it neither pools at the floor like propane nor collects at the ceiling like smoke. It mixes and travels with air currents and with the warm gas it arrives in, which is why manufacturers' instructions differ and why there is no single satisfying answer. Follow the instructions for your specific unit, keep alarms out of dead air within about 6 in of a ceiling-wall corner, and put them where you will hear them. Where to install CO detectors works through each room.

Who should not buy any of these

If you have no working smoke alarms, buy those first. It is not close: smoke alarms are the single highest-impact purchase in home fire safety, and a house with CO detection and no smoke detection has its priorities inverted. The smoke alarm roundup.

If you want CO and smoke detection from the same ceiling positions, buy combination alarms instead of separate units — fewer devices, fewer dates to track, one thing to replace. The trade-off is that a combination unit is replaced when either sensor ends its life. Combo versus separate works it through.

If your concern is chronic low-level exposure rather than acute poisoning — because someone in the household is pregnant, an infant, elderly or has a cardiac or respiratory condition — a UL 2034 alarm will not do what you want, by design. Buy one anyway, and add a low-level monitor. Best low-level CO monitors.