Clogging

• A heater core contains piping which is used to transmit hot air. This piping includes several bends in order to work in a constricted area. The piping can clog around the area of these bends if the car's coolant system is not flushed or changed on a regular basis. Clogged piping can result in lost heating capacity, and possible failure of the heater core.
  
  

Leaking

• The hosing that connects the heater core is used to transmit coolant.These connections can leak, causing coolant to fog the windshield above the windshield heater vent or even spill into the car directly. Leaking heater core connections can lead to heater core failure.
  
  

Corrosion

• Electrolysis, or the passing of an electronic current through a substance, helps power the heater core. This process can lead to corrosion, which can ultimately lead to heater core rupture and failure.