My IVT heater came with the house when we took over, so we don't know exactly how old it is the unit, but it is still fine visually on both indoor and outdoor unit.

Now the heating function does not work. The temperature only drops from 16 to 6 degree C within a short time and it goes silent but still lights up on the operation button.
It also happened with the Auto function.
Cold function is fine. Temperature cycle went up to 21/22 degrees and I can  feel cold air.

Have I made a mistake in the position? Or is it time to get rid of the heat pump? 

Thanks in advance to everyone who can give me advice.