Why does the car air conditioning suddenly stop blowing hot air?

[NolanFitz]

There are three main reasons why a car's heating system may not produce hot air: 1. The thermostat is stuck open or opens too early: The cooling system enters the major circulation cycle prematurely. With low external temperatures, especially when the car is moving, the cold air quickly cools the antifreeze, preventing the engine coolant from reaching the proper temperature and resulting in no hot air. 2. Damaged or slipping water pump impeller: Insufficient flow through the heater core leads to inadequate heat transfer. 3. Air lock in the engine cooling system: Air locks disrupt proper coolant circulation, causing high coolant temperature and failure of the heating system. Replacing the heater core can resolve this issue.