How often should you replace your CO detector unit?

CO sensors expire and stop detecting after ~5-7 years.

An expired CO alarm gives false reassurance. It will still light up and still respond to the test button while its ability to actually detect carbon monoxide has degraded.

1. Find the replace-by date. Printed on the back or the side. If there is no date, the unit predates the requirement and should be replaced now.
2. Buy to the current standard. Check the unit carries the relevant national standard mark for your country.
3. Consider a ten-year sealed unit. These remove both the battery task and the ambiguity about age, since the unit simply expires as a whole.
4. Place it properly. Roughly head height, one to three metres from the appliance, not in a cupboard, not directly above a heat source, and not in a bathroom.
5. Write the install date on it. And record the replace-by date somewhere you will actually see it.

• A repeating end-of-life chirp that a new battery does not silence
• A replace-by date that has passed
• A unit with no date, which is by definition too old

• Treating the end-of-life chirp as a faulty battery and cycling batteries through a dead alarm
• Buying an uncertified alarm online: check for the national standard mark rather than the price
• Placing it in a kitchen or bathroom, where humidity and cooking fumes shorten sensor life and cause nuisance alarms

• Combination smoke and CO units often expire earlier than a standalone smoke alarm, so track them separately
• Sensor life is shortened by high humidity and by exposure to solvents and aerosols
• Replace after any activation involving a genuine CO event

Replacing the alarm is a homeowner job. Working out why one sounded is not: that needs a Gas Safe, licensed or certified technician depending on your country.