Yes, plug-in carbon monoxide detectors go bad. They have a lifespan of around 7 years, after which the internal sensor loses its ability to accurately detect carbon monoxide. This is true regardless of whether the unit is hardwired, plug-in, or battery-operated. A detector that looks perfectly fine on the outside can be functionally useless on the inside.
Why CO Detectors Stop Working
Carbon monoxide detectors rely on an electrochemical sensor, a small chemical cell that reacts when CO molecules reach it. Over time, the reactive materials inside that cell degrade. The tiny particles that create the sensor’s active surface gradually dissolve or lose their electrochemical surface area, which means the sensor becomes less and less responsive to carbon monoxide in the air. This process is slow and invisible. You won’t notice any physical change in the device, and it won’t suddenly stop powering on. It simply becomes unreliable at detecting the gas it’s supposed to detect.
This chemical degradation is unavoidable. It happens whether or not the detector ever encounters carbon monoxide. Heat, humidity, and normal air exposure all contribute. By the time a unit hits the 7-year mark, the sensor may respond too slowly or not at all to dangerous CO levels.
How to Tell If Yours Has Expired
Every CO alarm manufactured after August 2009 is required to include an end-of-life warning signal. This is a distinct alert that tells you the unit needs to be replaced entirely, not just have its batteries swapped.
CO alarms produce three different signals, and it’s worth knowing the difference:
- Alarm signal: A loud, continuous pattern that means carbon monoxide has been detected. Leave the area.
- Trouble signal: A short beep (about half a second) every 30 to 60 seconds. This typically means a low backup battery or a fault.
- End-of-life signal: This may sound similar to the trouble signal but is often accompanied by a visual indicator, like a specific LED pattern, on the unit itself. It means the detector is done and no amount of battery replacement will fix it.
The trouble signal and end-of-life signal can sound nearly identical on some models. If replacing the backup battery doesn’t stop the chirping, your detector has almost certainly reached its end of life. Check the back or side of the unit for a manufacture date or expiration date. If it’s older than 7 years, replace it.
The Test Button Won’t Catch This Problem
Pressing the test button on your CO detector confirms that the alarm horn works and the internal electronics are functioning. It does not check whether the sensor can still detect carbon monoxide. A detector that passes the test button check with a loud beep can still have a degraded sensor that would fail to respond to actual CO in your home. This is why expiration dates matter more than test results for older units.
That said, you should still press the test button monthly to make sure the alarm circuit itself hasn’t failed. It’s a useful check for electrical problems. It’s just not a substitute for replacing an expired unit.
Maintaining a Plug-In Detector
Plug-in CO detectors have a backup battery (usually a 9-volt) that keeps the unit running during a power outage. That battery should be replaced once a year. If the backup battery dies and you lose power, you’ll have no carbon monoxide protection at all during the outage, which is exactly when CO risk can spike if you’re using a generator or alternative heat source.
After replacing the battery, press the test button to verify the unit is working properly. Some models will continue to chirp with a low-battery warning until the device runs its own internal battery check, so don’t be alarmed if it takes a short time for the chirping to stop after a fresh battery goes in.
Dust can also accumulate over the sensor vents on a plug-in unit, especially if it’s near the floor or in a kitchen. A quick wipe or light vacuuming around the vents every few months helps keep airflow to the sensor clear.
When to Replace It
Look at the back of your detector for a date. Some units print an expiration date directly. Others print a manufacture date, in which case you should add 7 years to determine when it needs to go. If there’s no date at all, the unit is old enough that it predates the 2009 labeling requirements, and you should replace it immediately.
When shopping for a replacement, look for a unit that’s listed to UL 2034, the safety standard for carbon monoxide alarms. This is a federal requirement for CO alarms, and any reputable brand will display the UL mark on the packaging. Plug-in models with a sealed lithium backup battery (instead of a replaceable 9-volt) are also available now, which eliminates the annual battery swap, though the entire unit still needs replacement at the end of its sensor life.
If you have multiple CO detectors in your home and you bought them all at the same time, they’ll all expire around the same time. It’s worth staggering replacements or at least marking your calendar so you’re not caught with a house full of dead sensors.