What is the reason for BMW having heating in the front but no heating in the rear?

[McAbel]

BMW having heating in the front but no heating in the rear is due to the following reasons: Thermostat always open or opening too early: If the thermostat is always open or opens too early, the cooling system prematurely enters a large circulation cycle. With low external temperatures, especially when starting the car, cold air quickly cools the antifreeze, resulting in lower engine water temperature and ineffective heating. Damaged or slipping water pump impeller: A damaged or slipping water pump impeller reduces the flow rate through the small heater core, causing slow heat buildup. Air lock in the engine cooling system: An air lock causes poor circulation in the cooling system, leading to high water temperature and ineffective heating. If there is always air in the cooling system, it may be due to a damaged cylinder gasket allowing air into the cooling system. If the inlet pipe of the small heater core is very hot while the outlet pipe is relatively cool, this indicates a blockage in the small heater core, which should be replaced.