Carbon monoxide is odorless, colorless, and deadly. The First Alert CO400 doesn’t try to be smart or fancy. It does one thing—detect CO—and does it reliably. That matters because a faulty furnace or a blocked chimney won’t announce itself. This two-pack covers the essentials: one for the basement near the furnace, one for the hallway outside bedrooms.

The battery-powered design is refreshingly simple. No electrician, no hardwiring, no app setup. You open the unit, snap in the included 9-volt battery, and mount it on a wall or set it on a shelf. The electrochemical sensor is the same technology used in professional-grade detectors. It picks up carbon monoxide from multiple fuel-burning sources—gas stoves, water heaters, fireplaces—before levels become dangerous. The alarm hits 85 decibels, which is loud enough to wake you from a deep sleep.

There is a trade-off. This model has no digital display showing CO levels, only an alarm and a test button. If you want to see real-time readings, you’ll need to step up to a pricier model. But for straightforward, no-nonsense protection across two zones in your home, the CO400 gets the job done without fuss.