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  • 2-in-1 Carbon Monoxide and Natural Gas Alarm
  • 120-Volt AC Plug-In Alarm with Battery Back-Up
  • Back-up Battery Continues protection in the event of a power outage
  • 9-Volt Energizer Alkaline Battery Included
  • Power and Alarm LED's
  • Compact Plug-in Design
  • No Additional Mounting Required
  • Swift Installation Process
  • Microprocessor Software Eliminates Nuisance Alarms
  • 5 Year Limited Warranty
  • Meets New Laws & Legislations - This alarm exceeds industry standards, and is listed to UL Standard UL 217 and is California State Fire Marshal approved.
The USI Electric MCN400 Plug-In Carbon Monoxide and Natural Gas Smart Alarm is a 2-in-1 device that provides double the protection with a single installation. This dual-sensing alarm protects against two deadly household threats: carbon monoxide and natural gas (or methane). Perfect for new or replacement installations, the 120-volt AC alarm is suitable for houses, apartments and mobile homes. The compact plug-in design covers only (1) outlet for a fast and easy installation with no wiring required. This alarm features microprocessor intelligence to eliminate nuisance alarms, a single silence/test button to quiet a non-hazardous alarm and an end-of-service life warning that sounds when it's time to replace the battery (approximately 7 years from activation). Three LED colors display the status of the alarm, power, CO and gas in your home. The 9-volt alkaline battery (included) continues protection in the event of a power outage.

This alarm is ETL Listed to UL Standard 2034 & UL1484, California State Fire Marshal approved and Federal Housing Authority (FHA) approved to exceed industry standards. For maximum protection, install an alarm inside every bedroom, outside every sleeping area and on all levels of the dwelling. The USI Electric MCN400 Plug-In Carbon Monoxide and Natural Gas Smart Alarm comes with a 5 year limited warranty.

Recommended replacement for the following obsolete models:
- CD-9590

Carbon monoxide alarms are designed to alarm before the average healthy adult feels symptoms. Since you cannot see or smell CO, never assume it's not present. Underwriters Laboratories Inc. Standard UL2034 requires residential CO alarms to sound when exposure times and exposure levels of CO in the chart below. CO alarm levels are measured in parts per million (ppm) of CO over time (in minutes). The UL2034 Required Alarm Points are:
  • If the Alarm is exposed to 400 ppm of CO, it must alarm between 4 and 15 minutes.
  • If the Alarm is exposed to 150 ppm of CO, it must alarm between 10 and 50 minutes.
  • If the Alarm is exposed to 70 ppm of CO, it must alarm between 60 and 240 minutes.
Carbon monoxide is not heavier than air. A carbon monoxide alarm should be installed in a location where the alarm will stay clean, and out of the way of children or pets. Refer to the user's manual for specific installation requirements.
It is very important to install a carbon monoxide alarm outside each separate sleeping area, and for maximum protection, install one in every bedroom. Many states now require installation of a carbon monoxide alarm inside each bedroom. For added protection, placement of an additional carbon monoxide alarm at least 15-20 feet away from the furnace or fuel burning heat sources is recommended. Install carbon monoxide alarms a minimum of 10 feet from sources of humidity like bathrooms and showers. In multiple-story dwellings, install one carbon monoxide alarm on every level. If you have a basement, install a carbon monoxide alarm at the top of the basement stairs, and in each separate sleeping area.
Do not install carbon monoxide alarms in garages, kitchens, furnace rooms, or in any extremely dusty, dirty, humid, or greasy areas. Do not install alarms in direct sunlight, or areas subjected to temperature extremes. These areas include unconditioned crawl spaces, unfinished attics, un-insulated or poorly insulated ceilings, and porches. Carbon monoxide alarms should not be installed in outlets covered by curtains, heavy furniture or other obstructions. Do not install in turbulent air-near ceiling fans, heat vents, air conditioners, fresh air returns, or open windows. Blowing air may prevent carbon monoxide from reaching the CO sensors.
Product Details
Product Specs
Carbon Monoxide Alarms
Product Manuals
USI Plug-In 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup Video

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MCN400B customer reviews

Overall Rating

17 Reviews
16 customers recommend this product
USI Plug-In 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup Reviews
frank b
June 9, 2022
Verified Purchase
CO detector

Works great, easy to install, lasts for years.

Rhonda F
March 24, 2022
Verified Purchase
Outstanding Customer Service

I had a Universal Security Carbon Monoxide and Natural Gas Alarm for 7 years when it reached the end of its lifespan. I purchased a new replacement and had difficulty getting it to work properly. I reached out to the customer service department who promptly sent a replacement. The new one works great! The company stands behind their products and treat the customer with respect.

Albin P
March 2, 2022
Verified Purchase
Peace of mind

This plug in carbon monoxide, natural gas detector assures me peace of mind.

David L
February 27, 2022
Verified Purchase
USI Plug-in Carbon Monoxide & Natural Gas Alarm

The product arrived exactly as ordered and on time. It was very easy to plug it in to the wall and testing it was also very easy. It was reasonably priced and adds allot of safety to my home.

Nathan T
October 3, 2021
Verified Purchase
Installed easy, one of them defective

I purchased two 2 in 1 Carbon Monoxide and Natural Gas Smart Alarms. One of the CO/Gas detectors became defective after about three weeks, the CO sensor kept going off. There was clearly no CO in the area, and since I had two of the detectors, I swapped locations and the bad sensor still went off. I need a replacement for that one. Otherwise I was happy with the purchase and the ease of install.


Hello, Nahan. We're sorry to hear this. Please reach out to us here and we'll get this resolved for you.

Tammy R
April 19, 2021
Verified Purchase
Best alarm out there

The dual alarm is the best out there!

April 12, 2021
Verified Purchase
2-in-1 CO and Natural Gas Smart Alarm with Battery Backup

I purchased the USI Plug-In, 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup MCN400B for my son's new residence. I have a couple of them in my home. We both have natural gas appliances throughout heating, hot water, etc. and that always means the possibility of carbon monoxide if anything goes wrong. Or the possibility of a natural gas leak. Carbon monoxide CO is an odorless, colorless gas formed by incomplete combustion and displaces oxygen leading to poisoning and death. But with this Universal Security Instruments CO and Natural Gas Smart Alarm, I know that my family is protected from both of these dangers by this highly reputable and trustworthy company. That's why we have these and other Universal Security Instruments products. This CO and Natural Gas Smart Alarm is indeed an absolutely marvelous safety device and now that my son has his own residence, I want him to be afforded the same protection that the rest of my family has always had. This USI Plug-In, 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup is the very best anywhere. I think of it like this : have you ever had water leak in your home? Most people have at one time of another. Well, you could just as easily have a natural gas leak or a carbon monoxide leak in your home. At least with a water leak, the worst thing that can happen is your house gets flooded. But with a natural gas leak or a carbon monoxide leak, you might die in your sleep. That's why when my family is sleeping, I want the USI Plug-In, 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup to be watching out for us. While my family sleeps, this alarm is always on guard.

William Davitt
June 2, 2020
Verified Purchase
Very satisfied

Works well and easy to use.

Jacob Eddy
January 6, 2020
Verified Purchase
Carbon/gas detector

Works as advertised and shipped quickly

Porfirio De La Cerda
November 3, 2019
Verified Purchase

Very good

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