My carbon monoxide alarm is beeping or chirping: what does it mean?

Carbon monoxide (CO) has no colour, smell or taste, so the alarm is your only warning, and one small speaker has to say three things: there is CO in the air, the battery is low, or the sensor is worn out. Since January 1, 2026 the Ontario Fire Code requires a CO alarm on every storey of a home with a fuel-burning appliance, fireplace or attached garage, so many homes now have more alarms, and more chirps, than before.

What do the different CO alarm beeps mean? Narrow it down

The rhythm tells you which message it is. Patterns vary by brand and model, and the label on the back of the alarm lists them; these are the ones Kidde and First Alert publish.

How do I tell a real CO alarm from a low battery?

By loudness and rhythm. A real alarm is loud and repeats in groups of beeps with short pauses; a battery warning is one short chirp, then about a minute of silence. If you are not sure, go outside first and work it out from the driveway. The test or silence button does not clear the air: on many models it quiets the alarm for a few minutes, then it sounds again if CO is still there. Low-battery chirps often start in the early hours, when the house is coolest and a weak battery is at its weakest.

Why is my CO alarm still chirping after I changed the battery?

Most often because the alarm has reached its end of life, and a new battery cannot fix that. Current alarms are usually rated for 7 to 10 years, though First Alert gives 5 to 7 for some models; the manufacture or replace-by date is on the back. Kidde notes that an alarm in end-of-life mode no longer responds to CO, so replace it with a new alarm certified to CSA 6.19. Check the simple things first: a loose battery, a pull-tab left in or a battery door not fully shut can also keep it chirping.

What should I do after the CO alarm goes off?

1. Stay outside until firefighters say it is safe. They usually carry gas meters and can tell you whether CO was present.
2. Do not use the furnace, water heater, fireplace or other fuel-burning appliances until they are checked. If firefighters or Enbridge shut off or tag an appliance, leave it off.
3. Have a TSSA-registered fuel contractor inspect every fuel-burning appliance and its venting. The TSSA's COSafety.ca site has a search for registered contractors.
4. Anyone who had symptoms should get medical advice, even if they feel fine outside.
5. If the alarm has a display or peak-level memory, note the reading before you reset or replace it.

Why do CO alarms go off more in the fall?

Because the sources wake up together. A furnace that sat all summer can start the season with a cracked heat exchanger, a loose vent joint or a flue blocked by debris, and the first cold nights are when it runs long enough for CO to build. Fireplaces are lit again, and cars warmed up in an attached garage push exhaust into the house. Later, snow can bury the plastic intake and exhaust pipes on the side wall, so clear them after every storm.

In Toronto and York Region homes, what should I know?

Enbridge Gas supplies natural gas in Toronto, Vaughan, Richmond Hill, Markham, Newmarket and Aurora, so most homes here have at least one gas appliance. Local patterns that matter after an alarm:

• Older houses: 29% of Toronto's homes, and about one in ten of Newmarket's, were built in 1960 or earlier (Statistics Canada, 2021 Census). Where the furnace was upgraded but an older gas water heater still vents into the original chimney, that venting is the first thing to check.
• Newer houses: about half of Vaughan's homes and about 44% of Richmond Hill's were built in 2001 or later. Their furnaces usually vent through plastic pipes on a side wall, so keep snow, leaves and shrubs clear of them. Builder alarms are often hardwired and linked; replace an expired one with a compatible model.
• Condos: almost half of Toronto's homes are in buildings of five or more storeys. In a unit, still leave and call 911, then tell building management, because the source may be in a shared garage, boiler room or another suite.

What should I tell the gas technician?

• Which sound the alarm made, which alarm it was (floor and room), and the reading if it has a display
• What the fire department found, and whether anything was shut off or tagged
• What was running at the time: furnace, water heater, fireplace, dryer, stove, or a car in the garage
• Whether anyone felt unwell, and whether it was worse at certain times of day
• The age of the furnace and water heater, and whether they vent through plastic pipes on a side wall or a metal pipe into a chimney
• Photos of the appliance labels and the vent pipes outside

What does a furnace safety check cost on Doyo?

The alarm itself is a 911 call; Doyo is for the inspection afterwards, once firefighters have cleared the house. Furnace Maintenance on Doyo is $201.25, the full price with Doyo's fee included. Say the CO alarm went off when you book, so the technician looks closely at the heat exchanger and venting. A part that needs replacing is Furnace Repair, priced by written quote before work starts. Doyo checks the technician's TSSA gas certificate before they are offered heating work.