Can this be Legal?

Hello,

 My parents rent a duplex, and have been living there for 9 years. They are just now getting the furnace checked for the first time. The company that came to inspect, that my parents offered to pay for will not touch the furnace as to it being in such poor condition. It is having CO2 "blasts" and the ducts are narrowed. My question is, can this be neglegence? Can my parents sue? They are now upset and confused, and are probably going to be looking for a new place to live. But is a landlord responsible for maintenance on such a thing???? My parents are in their 60′s and are now worried for their safety and health….