Is the landlord liable for tenants (ER visit ) insurance deductible due to a furnace breakdown resulting in high CO count?

The answer depends on whether the landlord had reason to know the furnace was malfunctioning or was about to malfunction. In Illinois, a landlord has a duty to inform a tenant of hazards the landlord should reasonably know about. Here, if the landlord did his due dilligence and checked the furnace according to its owner's manual as a reasonable landlord should, then liability most likely won't attach. However, if the landlord did not conduct a reasonable inspection of the premises, or knew about the problem and failed to warn the tenants, then liability may very well attach.

Note that the written lease can change the "duty of care" a landlord owes to his tenants to an extent, subject to state and local laws. The lease may change the above analysis.

NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.