P-306 HDI heating system

I have a P-306 Boulevard 2.0 HDI, I am new in these lides and I hope not to put the leg, I have a problem and I need a help. Thanks in advance. I don't get hot air, I think it is the control that does not open the passage of water well, because when I turn the control from cold to hot, it, it goes back a little, the fan flow is correct, and the level of the refrigerant is normal. How can I repair it?. Thank you again, a greeting.