How often should you replace a carbon monoxide detector?
Updated July 2026
Replace a carbon monoxide detector every 5 to 7 years, or by the sooner of the expiration date and the 10-year mark printed on the back of many newer models. This is not about the battery: the electrochemical sensor that actually smells CO wears out and simply stops responding, alarm or no alarm. Since carbon monoxide is odorless and colorless, an expired detector is worse than none, because it looks like protection you do not have. Replacements are cheap at $20–50 per unit.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Standard plug-in or battery CO alarm | Every 5–7 years | Typical electrochemical sensor service life |
| Unit stamped "replace by 2033" (10-year) | By that date | Newer sensors and sealed batteries are rated longer |
| Combination smoke/CO alarm | Every 10 years | Governed by the smoke sensor, whichever expires first |
| Alarm chirping a specific end-of-life pattern | Replace now | Many units beep to say the sensor, not the battery, is done |
| No manufacture date visible | Replace now | If you cannot date it, assume it is past due |
The cost of skipping it
What skipping it costs: the stakes here are not a repair bill, they are people. Carbon monoxide from a cracked heat exchanger, a backdrafting water heater, or a generator run too close to the house kills hundreds of Americans a year and sends thousands to emergency rooms, per the CDC. A working detector is the only warning you get; an expired one gives none. Against that, a $20–50 alarm every 5–7 years is the cheapest life-safety device in the house. Put one on each level and outside every sleeping area.
How to do it (5 minutes · DIY, no tools for plug-in models)
- Find the manufacture date on the back of each detector; the clock starts there, not on your install date.
- Buy the same type you are replacing: plug-in with battery backup, sealed 10-year battery, or hardwired and interconnected.
- For plug-in units, unplug the old one and plug in the new one; for hardwired, kill the breaker first and match the wiring harness.
- Press the test button until it alarms to confirm it works.
- Write the replace-by date on the housing with a marker, and log the fresh 5–7 year clock.
- Place units on every level and within 10 feet of each bedroom, but not right next to fuel appliances where they nuisance-trip.
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 replacements schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
What is the difference between replacing the battery and replacing the whole detector?
The battery powers the alarm; the sensor detects the gas. Swapping a dead battery on a good detector keeps it running, but no battery revives a sensor that has aged out at 5–7 years. That is why manufacturers print a hard expiration date: even a detector that tests fine and has a fresh battery can fail to alarm on real CO once the electrochemical cell degrades. When the replace-by date passes, the whole unit goes, battery or not.
How do I know if my CO detector is expired?
Check the back for a manufacture date and, on newer units, a printed replace-by date; count 5 to 7 years, or 10 on rated models, from manufacture. Many detectors also chirp a distinct end-of-life pattern, different from the low-battery chirp, that repeats even after you change the battery. If there is no date at all, it is old enough that you should replace it on principle. A $20–50 unit is not worth guessing about.
Where should carbon monoxide detectors go in the house?
Put one on every level, including the basement, and one within about 10 feet of each sleeping area so it can wake you. Keep them 15 to 20 feet from fuel-burning appliances like the furnace and water heater to avoid nuisance alarms, and off the ceiling directly above a stove. Attached-garage homes benefit from one near that connecting door. See how often to test them once they are up.
Are combination smoke and CO detectors a good idea?
They save wall space and wiring and are convenient near bedrooms, but there is a catch: the two sensors age on different clocks. Combo units are generally governed by the 10-year smoke-sensor life, while the CO cell may be rated 5 to 7, so follow the shorter number printed on the unit. Mount height also differs, since smoke rises and CO mixes evenly, so a combo is a compromise placement. For a furnace room, a dedicated CO alarm is often the better call.