Try disconnecting the electric (aux) water pump that supplies water to heater core durning idle & low RPM conditions. Any short circuit in that circuit will send the A/C into fixed operational mode (FOM). Also the windings in the mono heat valve will do the same thing.
If the incar temp sensor goes open or short you will have the same results.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR