[ITSv40]

I think it is most likely a flap issue. The cold air side is working correctly so that would suggest the sensors that relay temperature info must be working for the cold air system to know what to do. The same sensors are used for both hot and cold - that is assuming you have the air con engaged. Heat comes from the engine coolant circulating through the heater matrix - so that is either going to be hot or cold depending on water flow, there are no control valves in the water flow. Having got heat into the matrix there is only one way to control it and that is flaps controlling the airflow - either letting air flow over the heater matrix or not. These flaps are controlled by the climate unit - by working out whether cold air or warm is needed to achieve the desired temp set by the driver.

Looking on Vida there is a flap motor, passenger side of the central air/heater box high up under the dash. I think it is most likely this motor is not getting the signal/electric supply, is not working once it does get current and or the flaps themselves are disconnected/jammed in their housing.

Turning the dial from 15 to 25 should change the airflow from cold to hot almost instantly. Sorry I cannot be more specific, but hope it helps.