First you need to check with your owners manual for the exact control funtions. If you don't know how to control it there is no hope for diagnosing it.
What you will eventually want to know is whether you have a refrigeration, control, or heater problem. You should see if the compressor is coming on. You should see if the heater valve is functional. It is vacuum controlled. Turn it off and on and see if the arm moves under vacuum control. With this info you can ask the right questions. That system is fairly complicated - let us know what you have going.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician