Has anyone else received the CO400 with one 9 volt battery instead of the two AA batteries listed in description and on picture of packaging display?

Yes, Received the 9-Volt battery version. I live in California. I'm not a lawyer but my research shows that in California Smoke detectors and Combo Smoke/Carbon detectors must come with a 10 year lifespan battery. This law does not seem to apply to devices that only detect Carbon Monoxide like that one for sale here.

Why can't this be purchased in or shipped to California?

I live in California. Yes, I received the 9-Volt battery version. I'm not a lawyer but my research shows that in California Smoke detectors and Combo Smoke/Carbon detectors must come with a 10 year lifespan battery. This law does not seem to apply to devices that only detect Carbon Monoxide like that one for sale here.

the package picture shows 9-volt, but sounds like it is a crapshoot on what they will send you. which is better and lasts longer, 2AA or 1 9V? I hate taking out and putting in 9v batteries. so many to choose from. would prefer plug-in but would be too low.

I bought a package of AA batteries because this item description said it takes 2 AA. It does not. It takes (and comes with) one 9V. This is model CO400.

Red light is on. Is that normal? Tested and it chirps with test. Also checked battery.

YEs, red light is normal. If red light blinks - read manufacturer's guide in link below. https://www.brkelectronics.com/pdfs/2017/10/23/803bf30c.pdf

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Description

🚨 Stay ahead of danger with the CO400 — because your peace of mind can’t wait!

EFFORTLESS SETUP - No wiring needed—place it anywhere with optional wall mount for instant peace of mind.
LOUD CLEAR ALERTS - 85-decibel alarm guarantees you’ll hear danger before it’s too late.
PRECISION DETECTION - Electrochemical sensor delivers the most accurate carbon monoxide readings available.
SMART END OF LIFE SIGNAL - Built-in chirp notifies you when it’s time to replace the unit, so safety never lapses.
UNINTERRUPTED PROTECTION - Battery operation ensures continuous CO monitoring—even during power outages.

The First Alert CO400 is a UL-listed, battery-operated carbon monoxide detector featuring a 5-year electrochemical sensor, an 85dB alarm, and smart end-of-life alerts. Designed for easy placement without wiring, it ensures reliable, continuous CO monitoring to protect your home and loved ones—even during power failures.

Specifications

Brand	First Alert
Style	Battery Powered
Power Source	Battery Powered
Color	White
Product Dimensions	1"D x 5"W x 3"H
Item Weight	7.2 ounces
Alarm	Audible
Operating Humidity	10-95% RH
Sensor Type	Electrochemical
Global Trade Identification Number	00029054000699
Material	Multi
Number of Items	1
Manufacturer	First Alert
UPC	029054000699
Part Number	CO400
Item model number	CO400
Batteries	2 AA batteries required. (included)
Is Discontinued By Manufacturer	No
Size	Pack of 1
Pattern	Detector
Voltage	120 Volts
Item Package Quantity	1
Number Of Pieces	1
Certification	UL2034
Included Components	Alarm; Mounting Bracket; User Manual; 9V Battery
Batteries Included?	Yes
Batteries Required?	Yes
Battery Cell Type	Alkaline
Description Pile	9V battery
Warranty Description	5 Year Limited Warranty

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