Current code favours a sealed, 10-year alarm over the kind you replace batteries in, so an older 9-volt unit is worth swapping before your possession date rather than after. Joel Dyck flags any missing or expired alarm during a pre-listing walkthrough so it never becomes a last-minute possession-day problem. The requirement is not just a listing nicety, because a lawyer handling possession day paperwork will ask whether alarms are present and working, and a seller who cannot answer that honestly is creating a problem for the buyer's insurance rather than a cosmetic one.
A carbon monoxide alarm matters most in homes with a gas furnace, gas water heater, fireplace or attached garage, since that is where carbon monoxide actually originates, and it should sit near sleeping areas rather than buried in a utility room where no one will hear it.
Testing every alarm and replacing anything expired before your possession date is a small task that avoids a real delay. Get your hand-reviewed valuation from Joel Dyck.