Customer Reviews For The USI Plug-In 2-in-1 CO and Natural Gas Smart Alarm with Battery Backup

Overall Rating

17 Reviews
16 customers recommend this product
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

Carol Wesley
September 24, 2019
Verified Purchase
Two ways

I like that this device detects both CO and natural gas now that our home has changed from oil to gas heat. The back up battery is also a comfort.

Carol Wesley
September 24, 2019
Verified Purchase
Two ways

I like that this device detects both CO and natural gas now that our home has changed from oil to gas heat. The back up battery is also a comfort.

August 23, 2019
Verified Purchase
2-in-1 Gas Alarm

- Very compact - Easy to order - Easy to install, just plug it in - Great price I have four units in my home and will return to Universal when I need replacements.

Kristin Pardue
May 2, 2018
Verified Purchase
Quick and Easy

Quick and easy safety.

Sam Stierman
February 14, 2017
Verified Purchase
Plug-In 2-in-1

Easy to install--pull the battery tab and plug it in.

Robert Norwillo
December 14, 2016
Verified Purchase
2 in 1 Alarm

A combination safety device which was a cinch to install and test. I simply trust that it does not alarm as it is designed to do. Thank you.

John  Lelli
October 26, 2016
Verified Purchase


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.
Having a home natural gas detector or alarm is crucial to ensure your safety, as leaks can originate from various parts of your plumbing. Interior gas lines, which transport gas throughout your house to appliances, can develop leaks due to corrosion, improper installation, or wear and tear. The appliances themselves, like furnaces, water heaters, and ovens, can also be culprits, leaking from faulty valves, hoses, or connections, especially if not maintained properly or if the appliances are old. Even underground pipes bringing gas to your house from the main line can leak due to accidental digging or events like earthquakes.
To ensure your home natural gas detector keeps you safe, replace it every seven years, even if there are no warning signs. Check the expiration date on the detector itself, but plan for replacement every seven years regardless. A natural gas detector will beep frequently (every 30 seconds to a minute) or display an "EOL" (End-of-Life) alert when nearing the end of its lifespan, but don't wait for these warnings before replacing.
A beeping natural gas alarm can indicate either a dangerous gas leak or a less serious issue. Evacuate your home immediately and call 911 from outside if the beeping is continuous, as this signifies a potential leak. However, a natural gas leak detector alarm may use short, regular beeps (every minute or so) to signal low battery or the end of the detector's life.
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.
When natural gas leaks into a confined space, it can build up to an explosive concentration. If there's an ignition source like a spark from an appliance or a lit cigarette, an explosion can occur.

The Universal Security Instruments Electric MCND400B Plug-In Carbon Monoxide and Natural Gas Smart Alarm provides dual protection against two deadly threats: carbon monoxide and natural gas. This innovative 2-in-1 device simplifies installation while safeguarding your home and family. The compact plug-in design conveniently fits into any standard outlet, requiring no professional installation. This makes the alarm ideal for homes, apartments, or mobile homes.

  • Carbon Monoxide Detection: The 2-in-1 alarm utilizes microprocessor intelligence to analyze carbon monoxide readings and minimize false alarms. This ensures reliable detection while reducing unnecessary disruptions.
  • Natural Gas Detection: The unit also operates as a natural gas detector, providing early warnings of potential hazards.
  • End-of-Service Life Warning: This 2-in-1 carbon monoxide and natural gas detector provides an alert when it reaches the end of its lifespan (approximately 7 years from activation). A 9-volt alkaline battery (included) continues protection in the event of a power outage.

This USI Carbon Monoxide and Natural Gas 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 Universal Security Instruments 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

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