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🍁 Built for Canadian homes — written to Canadian codes and provincial rules. Not intended for use outside Canada.
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Carbon monoxide alarm is going off

First, the important part: a sounding CO alarm is treated as real until proven otherwise. If it's the steady alarm — or anyone feels unwell — get everyone outside and call 911. Sort out the cause after you're safe.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
emergency_home

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Start here

In Canada, treat a steady CO alarm as real. Get everyone and any pets outside into fresh air right now, and call 911 or your fire department from outside — do not go back in to investigate or open windows first. After it's confirmed safe, a licensed HVAC/gas technician finds the source before anything is used again.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

verifiedWhy this one goes to a pro

After you're safely outside and emergency services have cleared the home, the source of carbon monoxide — a cracked heat exchanger, a blocked vent, a failing water heater, or exhaust from an attached garage — must be found and repaired by a licensed HVAC technician or gas fitter before any fuel-burning appliance is used again. Do not run the furnace or water heater until it's cleared.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

visibilityHow to tell a chirp from an alarm

report If it's the steady alarm, do not troubleshoot — leaveNever try to diagnose a sounding CO alarm from inside the home. If it's the loud, repeating alarm, or anyone feels unwell, get everyone and pets outside into fresh air and call 911. Only an intermittent chirp, with no one feeling ill, is safe to investigate — and even then, when in doubt, get out.
  1. From a safe spot, listen to the pattern. A single short chirp every 30–60 seconds is a low-battery or fault warning.
  2. A CO alarm is loud and continuous or repeats a set of four quick beeps over and over — if that's what you hear, stop reading and get out.
  3. Only if it's the intermittent chirp (and no one feels unwell) is it safe to change the battery or check the alarm's date.
buildNo tools schedule2 min

handymanHow to fix it

Once you've confirmed it's a low-battery chirp (not the alarm) and no one feels unwell, replace the battery with the exact type marked on the unit and press Test.

  1. Confirm it's the intermittent chirp and that no one has symptoms — otherwise leave and call 911.
  2. Open the battery compartment and fit a fresh battery of the exact type printed on the alarm.
  3. Close it, remount, and hold Test until it sounds. If it keeps chirping with a fresh battery, check the date — it may be at end of life.
schedule5 min savings$4–12
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Replacing a CO alarm battery is homeowner maintenance. Finding/repairing a CO source is licensed HVAC/gas work in every province.
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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.