How often should you test a carbon monoxide detector?
Updated July 2026
Test every carbon monoxide alarm once a month with its button, and put a fresh battery in each battery-powered unit once a year. Because CO is invisible, odorless, and gives no physical warning until you are already sick, the detector is your only line of defense, and a silently dead one is the whole distance between a loud alarm at 3 a.m. and a tragedy. The monthly press takes seconds per unit and confirms the horn and electronics still respond.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| All CO alarms, routine check | Monthly | Confirms power and horn on each unit |
| 9V or AA battery units | New battery yearly | A dead battery is the top cause of a silent alarm |
| Sealed 10-year battery units | Test monthly, no swap | Battery and sensor retire together |
| Hardwired with battery backup | Test monthly, backup yearly | Backup covers outages when CO risk rises |
| Start of heating season | Test all before first furnace run | Combustion appliances are the main CO source |
| Alarm sounds for real | Get to fresh air, call 911 | Treat every CO alarm as real until cleared |
The cost of skipping it
What skipping it costs: unlike most home maintenance, the downside here is not a dollar figure, it is your family. Carbon monoxide from a cracked heat exchanger, a blocked flue, a backdrafting water heater, or a car left running in an attached garage displaces oxygen in the blood, and the early symptoms (headache, dizziness, nausea) are easy to mistake for the flu. The CDC ties CO poisoning to hundreds of US deaths and thousands of ER visits each year, and a detector you never test may be dead when it matters. Testing is free; replace any unit that fails the test or is past its 5–7 year life, at $20–50 each.
How to do it (2 minutes monthly · DIY)
- Press and hold each alarm button until it sounds its full alarm pattern; a weak or missing tone means replace the unit.
- On battery models, change the battery once a year and test immediately after.
- Note the digital display if the unit has one; a resting reading should be 0 ppm in clean air.
- Never test by exposing an alarm to actual exhaust or a flame; the button test is the safe check.
- Confirm placement: one on each level and near sleeping areas, 15 to 20 feet from fuel appliances to avoid nuisance trips.
- If any unit is past its printed 5–7 year date, replace it rather than relying on the test.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing CO detector tests schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
What should I do if my carbon monoxide alarm actually goes off?
Treat it as real every time. Get everyone, including pets, outside to fresh air immediately, then call 911 or the fire department from outside. Do not go back in to open windows or hunt for the source; ventilating first can mask the problem and delay you. If anyone has a headache, dizziness, nausea, or confusion, tell the dispatcher, since those are CO symptoms. Let the fire department clear the home and identify the source, often the furnace or water heater, before anyone re-enters.
How is testing different from replacing a CO detector?
The monthly button test proves the horn, battery, and circuitry still work, which catches the most common failure, a dead battery, right away. What it cannot do is prove the electrochemical sensor still detects gas, and that sensor expires on a fixed 5-to-7-year clock, 10 on some rated models. So testing and replacement are two separate habits: test monthly to catch power failures, and replace the whole unit on its printed date regardless of how it tests.
How often should I change the batteries in a CO detector?
For units with replaceable batteries, install fresh ones at least once a year; syncing it with a daylight-saving clock change or the start of heating season makes it easy to remember. Test the alarm right after every swap. If your detector chirps intermittently, that is usually the low-battery warning, so replace the battery that day. Sealed 10-year units skip this entirely: you never open them, and when the battery dies at end of life, you replace the whole detector.
Where should I place CO detectors so they test true?
Put a CO alarm on every level of the home and within about 10 feet of each sleeping area so it can wake you. Keep units 15 to 20 feet away from the furnace, water heater, and gas range to prevent nuisance alarms from normal startup, and out of dead-air corners and direct drafts. Attached-garage homes should add one near the connecting door. Good placement means your monthly test reflects real-world protection, not a spot that never sees air movement.