The heater only delvires cold air. What can that be?

The temperture of the car is good.
The airco works.
There is enough coolant in the car.
The buttons of the heater also works. You can hear the slide goes if you turn the button.
the hoses to the heater looks good.

Sorry for my english... I am a from he Netherlands!

Thanks for the answers!

Regards
Marc

 

anyone?

 

flush your cooling system, maybe have a plugged heater core?
also, there is something in the dash that sometimes stops working.... a flap or something... maybe a search will find it