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First Alert CO-400

Est. $20
June 2013
by ConsumerSearch
First Alert CO-400

Battery-operated carbon monoxide detector

  • Easy to install
  • Placement flexibility
  • Runs on two AA batteries
  • Loud horn
  • Battery door easily damaged
  • No CO level display

Bottom Line

The First Alert CO-400 is a basic battery-operated carbon monoxide detector that allows for placement flexibility. It is easy to use, compact and inconspicuous. In keeping with its no-frills design, the detector doesn't emit verbal warnings or provide a readout of current or past CO levels, but will simply alarm when it senses carbon monoxide. Some users find decoding its various chirps difficult, but aside from its finicky battery door, it doesn't have a lot of mechanical problems.


Effective. Owners like that the First Alert CO-400 is effective, simple to use, has many placement options and does not use up an outlet. It can be wall-mounted and is small enough to be set on a shelf. It's a good model for people who don't need or want the feedback given by the higher-end units. For example, it doesn't show current CO levels or store past levels in its memory. Because of this, many owners say they aren't 100 percent sure that it is working and rely on the blinking operation light and the test button as indicators.

Some owners find decoding the detector's chirps confusing, especially if they don't have the manual handy. A chirp every minute indicates a low battery, while a flashing red light with a four-beep horn pattern means the detector has sensed carbon monoxide. Three chirps indicates a malfunction signal, while five chirps means the detector has reached the end of its life, usually after five years. Like the other top-rated carbon monoxide detectors, the First Alert CO-400 meets Underwriters Laboratories (UL) standards.

Ease Of Use

Easy to install and test. This First Alert model is a bare-bones carbon monoxide detector with very few features, but its simplicity attracts many users. The unit comes with two AA batteries, two screws and two drywall anchors. When the batteries are first connected, the horn will chirp and the lights will blink once. After that, a light flashes every minute to show that it is receiving battery power. Owners find the instructions easy to read and appreciate the mounting template included in the manual. Some reviewers who owned previous models complain that their old mounting holes did not match up with the current model's, but this doesn't seem to be a widespread problem.

Owners like that the detector can be mounted on two screws that slide into holes on the back or can be placed on a shelf and moved when needed. Parents love having the option to move it out of curious children's reach. Testing is as easy as pressing a button and waiting for the sequence of four beeps, a pause and another four beeps. The horn is extremely loud, and owners advise wearing earplugs during testing.


Persnickety battery door. The batteries must be properly installed for the battery door to close. Several owners say they broke the door's latching mechanism after forcing the door closed without batteries and wish there was a proper warning included in the instructions. The broken door did not interfere with operation, and a few users simply taped the door shut. The unit does not take rechargeable batteries, but owners don't complain about constant battery changes. The First Alert CO-400 has an end-of-life timer and is designed to last about five years.

Where To Buy
First Alert CO400 Battery Powered Carbon Monoxide Alarm

Buy new: $34.99 $16.79   133 Used & new from $11.75

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Our Sources


More than 110 people reviewed the First Alert CO-400 on About 55 percent praised its simplicity and rated it 5 stars. Those who gave it 1 star (about 12 percent) complained about purchasing defective units or their units breaking soon after their purchase.

Review: First Alert CO400 Battery Powered Carbon Monoxide Alarm, Contributors to, As of May 2013


Nearly 50 owners reviewed the CO-400 detector on, giving the device an overall 4.6-star rating out of 5. There were only two reviewers who gave the detector 1 star.

Review: First Alert Battery-Powered Carbon Monoxide Detector, CO400, Contributors to, As of May 2013

3. sells several battery-powered carbon monoxide detectors. The First Alert CO-400 was reviewed by about 10 people, who gave the device a score of 4.6 out of 5 . The overall positive reviews were kept short and simple.

Review: First Alert Battery Powered Carbon Monoxide Alarm, Contributors to, As of May 2013

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