Why would a car heater stop blowing hot air when the fan is working?

Answer 1

If the car's cabin heater element is getting no feed of hot water from the engine, the heater fan can only blow cold air!

Sounds like you could have one or more different faults:

* A faulty coolant water thermostat: it may be stuck "always open" so any hot water from the engine block goes mainly to the car's radiator and hardly any is being allowed to reach the car's cabin heater. That is a very common cause of this problem in any car that is more than 10 years old. It should not be a big or expensive job to replace a faulty thermostat.

* There could be some air trapped (known as an air lock) or some other kind of blockage in the heating system which is preventing hot water from flowing from the engine block into the car's cabin heater element. The hose between the engine block and the heater element needs to be inspected for a physical blockage or an air lock.

* The engine's head gasket has blown slightly, allowing a small leak of exhaust gas to build up in the engine coolant. If that is what has happened another tell-tale sign - apart from the cold heater - is that some engine oil may be visible floating on top of the coolant water if the radiator cap is removed when the engine is cold. Any car mechanic should be able to check your car's heating system for the above faults. They are fairly common in any make of older car, especially if the engine coolant/anti-freeze has never been flushed out and renewed throughout a long running life.