I had this problem last year. It was solved by reconnecting a vac line that was near the oil filter. The line was not immediately apparent but it drops down in the oil filter area to a connector which can inadvertently pulled loose during a filter change. When I found the line, I pulled slightly on it and it was apparent it was loose. I think I had to reconnect it from under the car. When reconnected, everything worked normally again.
passenger side vents suddenly start blowing cold air when heat is on. Driver side vents still blow hot air.
A/C blows fine, but blower is stuck in defrost mode, and won't blow out the regular vents.
floor vent is blocked / does not open. all other positions are ok.
i like the heat coming from floor.
Poor operation of the air circulation and temperature when turn to the lowest or highest degree
had cracked vacuum block replaced but air only blows hot air
Air does not flow from the center vents. MB dealer said it was the air-conditioning actuator, which needs to be replaced. They said it is a $600 to $800 repair at the dealership and could be fixed for much less at an independent shop (recommended that due to the car's age and value, especially since its not a critical safety system).
Heat does not come through the vents when commanded, only cold air (AC) then as the car drives for about 1/2 hour or 45 min then the heat slowly starts to work better till its almost normal.