First Alert CO400 Review: Battery CO Alarm for Wood Heat

First Alert CO400 review: the short verdict

The CO400 is a battery-powered carbon monoxide alarm, model CO400, in white at 1 by 5 by 3 inches and 11.2 ounces. It uses an advanced electrochemical sensor that the listing says accurately detects CO levels no matter where the detector is placed in the room. When CO reaches dangerous levels, a loud 85-decibel alarm sounds; pressing the button should sound the alarm for 5 to 6 seconds. The box includes the alarm, a mounting bracket, a user manual and a 9V battery. The listing states a 5-year limited warranty, an end-of-life signal chirp that sounds when the unit needs replacing, and that all First Alert CO alarms meet UL standards.

The trade-offs are the battery and the single-sensor design: the 9V battery needs replacing, and there is no digital display of CO levels on this model. The listing recommends one alarm on each level of the home and in each bedroom.

Key specs

Specifications are manufacturer rated as listed, collected 19 October 2026.

The alarm

What the CO400 does well

Electrochemical sensor

The sensor is the part that matters: an advanced electrochemical carbon monoxide sensor, the same sensing technology used in professional instruments. The listing says it accurately detects CO levels regardless of placement in the room. Electrochemical sensors are the standard for residential CO alarms because they respond specifically to carbon monoxide rather than to humidity or temperature swings.

Battery power for outages

The CO400 runs on a 9V battery, included in the box, so it monitors continuously even during a power failure. That matters for wood heat: winter storms knock out power exactly when the stove is working hardest. The listing notes the battery is replaceable, and the sound check (5 to 6 seconds of alarm) confirms the battery and sounder on demand.

85-decibel alarm with end-of-life chirp

The alarm sounds at 85 decibels when CO reaches dangerous levels, loud enough to wake sleepers. A separate end-of-life signal chirp tells the owner when the unit’s sensor has aged out and the alarm needs replacing, so an expired alarm never silently guards the house. The listing backs it with a 5-year limited warranty.

UL standards, trusted brand

The listing states all First Alert CO alarms meet UL standards, and notes First Alert launched the first residential smoke alarm in 1958. For a life-safety device, the safety standard and the maker’s track record are the credentials that count.

Limitations to know before you buy

No digital CO display

The CO400 has no display showing current CO levels; it alarms or it does not. The listing describes an audible alarm only. Anyone who wants to see low-level readings needs a different model.

9V battery to maintain

Battery power is the feature and the chore: the 9V battery must be replaced on schedule, and a dead battery means no protection. The check button exists for exactly this reason; the listing says to check it regularly and replace the unit if it does not alarm.

One per level, per the listing

The listing recommends one alarm on each level of the home and in each bedroom for complete protection. A single CO400 covers one spot; a wood-heated house needs several.

5-year life, then replace

The 5-year limited warranty sets the planning horizon: when the end-of-life chirp sounds, the unit is done. Budget for replacement, not repair.

More details from the listing

The check routine, step by step

The listing spells out exactly how to confirm the alarm works: press the button and the alarm sequence should last 5 to 6 seconds. If it does not alarm, check that fresh batteries are correctly installed and press the button again. If the unit still does not alarm, replace it immediately. That is the whole maintenance routine on paper: a working button check, fresh 9V batteries, and replacement when the check fails or the end-of-life chirp sounds.

Box contents and model codes

The box holds the alarm, a mounting bracket, a user manual and a 9V battery, one count of each. The model number is CO400 and the manufacturer part number is CO400; the item type name adds an FF suffix, “First Alert CO400FF Battery Powered Carbon Monoxide Alarm”. The unit is white, 1 inch deep by 5 wide by 3 high, and 11.2 ounces, with the enclosure material listed as Multi. Those codes are what to match when comparing sellers or reordering.

The brand claims, reported as claims

Two brand statements ride along in the listing. First, all First Alert CO alarms are stated to meet UL standards; that is the safety credential, reported as the maker’s claim. Second, the listing calls First Alert the most trusted brand in home safety since launching the first residential smoke alarm in 1958, based on a First Alert brand trust survey from February 2018. The survey is the maker’s own, so read the trust claim as marketing rather than measurement.

How we scored it

Scores run from 9.2 to 9.8: each product’s point total is placed on that range, so more points always means a higher score.

The Firewood Score weighs performance, build, value, safety and warranty. The CO400 scores for its electrochemical sensor, battery backup, 85-decibel alarm and stated UL standards. No display and battery maintenance hold it back.

Who should buy the CO400, and who should skip it

Buy it if

• You heat with wood and want CO monitoring that survives power outages.
• You want a simple alarm with an end-of-life chirp and a 5-year warranty.
• You need several alarms, one per level and bedroom, at a reasonable cost.

Skip it if

• You want a digital display of CO levels; this model has none.
• You want a 10-year sealed battery; this uses a replaceable 9V.
• You want smart-home alerts; this is an audible alarm only.

Placement advice for wood-heated homes

Where the listing says to put them

The listing recommends one alarm on each level of the home and in each bedroom. For a wood stove home, that means the stove room, the hallway outside bedrooms, and each bedroom. The listing says the electrochemical sensor reads accurately wherever the detector sits in the room.

Check on a schedule

The listing describes the check: press the button and the alarm sequence should last 5 to 6 seconds. If it does not alarm, check that fresh batteries are correctly installed and press the button again; if it still does not alarm, replace the unit immediately. Regular checks are the habit that makes the alarm trustworthy.

Frequently Asked Questions

How loud is the CO400 alarm?

85 decibels, per the listing, when CO reaches dangerous levels.

Does it work during a power outage?

Yes. It is battery powered with a replaceable 9V battery, included in the box, so monitoring continues without mains power.

How long does it last?

The listing states a 5-year limited warranty, and an end-of-life chirp signals when the unit needs replacing.

Is it UL listed?

The listing states all First Alert CO alarms meet UL standards.

Where should I install it?

The listing recommends one on each level of the home and in each bedroom.

Does it show CO levels?

No. The CO400 is an audible alarm with no digital display.

Related guides

For more stove safety, browse our stove accessories and wood stoves guides.