If your home has a gas furnace, water heater, fireplace or an attached garage, Ontario law requires working CO alarms near sleeping areas. They expire — check the date on the back; most last 7–10 years.
Your furnace's role: an aging heat exchanger or blocked venting is the classic residential CO source. This is exactly what the combustion and heat-exchanger inspection in an annual tune-up exists to catch, years before it becomes dangerous.
If a CO alarm ever sounds: get everyone outside, call 911, and don't re-enter until it's cleared. Then have the fuel appliances inspected before running them again. This is one topic where 'probably fine' isn't a plan.
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