What is a carbon monoxide detector?
Updated August 2026
A carbon monoxide detector is an alarm that samples the air for carbon monoxide, the colorless and odorless gas any fuel-burning appliance can produce, and sounds off well before the concentration in the room turns dangerous. Inside sits an electrochemical cell that reacts with CO and generates a tiny current. Listed alarms respond on a sliding scale: 400 parts per million within 4 to 15 minutes, 150 ppm within 10 to 50 minutes, 70 ppm within 1 to 4 hours. Brief low-level exposure is tolerated on purpose, so cooking and traffic do not set it off.
Where it is
Put one on every level of the house, a finished basement included, and one within 15 feet of each sleeping area so it can wake people. Add one near the door to an attached garage, where a car warming up for two minutes can push a lethal dose indoors. Keep them at least 15 feet from the furnace, water heater, and gas range, because the small puff those appliances make at startup triggers false alarms. Mounting height barely matters: carbon monoxide mixes evenly with air rather than rising the way smoke does.
What it does
Carbon monoxide gives no warning of its own. It binds to hemoglobin hundreds of times more readily than oxygen does, and the early symptoms, headache, nausea, dull confusion, read exactly like flu, which is why people go to bed instead of going outside. The sources are ordinary: a cracked furnace heat exchanger, a blocked or disconnected flue, a fireplace with a closed damper, an unvented space heater, a generator running too close to a window. The alarm's own weakness is time. Electrochemical sensors drift and dry out whether or not they ever meet gas, and a unit past its date will read zero in a room full of CO. That is the case for refusing to treat these as set-and-forget.
Maintenance
Press the test button monthly to confirm the horn and electronics still work, and vacuum the vents twice a year; see the monthly test routine. Change replaceable batteries once a year. Then the part most people miss: the sensor itself expires, so the entire unit gets replaced every 5–7 years, or 10 for models rated that long, per the replacement schedule. Write the install date on the housing in marker the day you mount it, because the manufacture date printed on the back is a different number.
What it costs to replace
A replacement carbon monoxide detector runs $20–60 for a plug-in or battery alarm, or $70–120 for a hardwired combination smoke and CO unit.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Why does my CO detector chirp when the battery is new?
Because it is announcing that the sensor has expired, not that it is hungry. Most alarms use a distinct end-of-life pattern, often a single chirp every 30 seconds that continues after a battery swap, sometimes paired with a specific light or a spoken message. Check the label on the back for a manufacture date. Anything older than about seven years is finished, and no battery will quiet it. Replace the whole unit.
Do I need CO detectors in an all-electric home?
Usually yes. The question is not what heats the house but whether anything burns fuel near it: an attached garage, a wood or gas fireplace, a fire pit on a deck under a window, a portable generator, or a vented appliance in a shared wall. A detached-garage home with no combustion at all carries genuinely low risk. At $20–60 an alarm, most owners still put one outside the bedrooms and stop thinking about it.
What should I do when the CO alarm sounds?
Get every person and pet outside first, then call 911 or the fire department from outdoors. Do not open windows to air the house out before you leave, because that erases the readings responders need to locate the source. Do not silence the alarm and go back to bed, which is the decision that kills people. Once the house is cleared, have a technician inspect the furnace, water heater, and flues before anything fuel-burning runs again.
Can one alarm cover both smoke and carbon monoxide?
Yes, combination units are common and run $70–120 hardwired. The catch is mismatched lifespans: smoke sensors are rated for 10 years and CO sensors for 5 to 7, so the combo carries the shorter date and the whole device goes in the bin early. Some owners prefer separate alarms for that reason, plus the freedom to place the CO unit near bedrooms and the smoke unit well away from the kitchen.