Stand-up heating again

Good evening, everyone.
since the Webasto seems to be a favorite topic here, I too would like to ask a question about the same. You can say that from pretty exact + 4 degrees outside temperature, my preheater turns on but then goes out again. At above * 5 degrees everything normal, when engine warm everything normal and when activation of the heater by hand (so without timer) everything normal. What could that be??? I've got the Klimatronic operating part suspected of cold soldering or the vulture knows.
I noticed two funny things about my climatetronic control unit/control unit:
1: We all know that on the diagnostic channel number 20 you can read the instantaneous onboard voltage.
With me it's like this: I don't start the engine with the last engine switch off!!Deactivated air conditioning shows me this only 12.0 volts. I turn off the system with the blower control and get the correct onboard voltage again.
On the other hand, I switch off the climate before the engine is switched off, and the engine next time (with now disabled climate)
from the start, the correct on-board voltage is immediately displayed.
It is also the case with the external temperature indicator(have built in a new sensor to be sure) It rises with the engine shut down and ignition on ever higher and needs 3-4 seconds when starting the engine, too, before it is displayed at all.
So, could this indicate a defect in the ECU?
By the way, the control of the air conditioning works perfectly.
Maybe someone can tell me where the control unit is for the heater? I downloaded the Rep. guide(fire heater) but unfortunately I can't open it.
Thank you in advance, Martin.