The vacuum manifold for the climate control actuator may develop a leak, This can cause poor or no operation of the air delivery and temperature control flaps.Google+
Car Problem Reports
Mercedes-Benz C280 Control Flaps May Fail Due to Leaking Vacuum Manifold for Climate Control Actuator
Mercedes-Benz C280 Problem
Average Mileage: 110,729 mi (60,000 mi - 154,000 mi)
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.
Visitor, 1999 Mercedes-Benz C280
passenger side vents suddenly start blowing cold air when heat is on. Driver side vents still blow hot air.
Visitor, 1995 Mercedes-Benz C280
A/C blows fine, but blower is stuck in defrost mode, and won't blow out the regular vents.
Visitor, 1996 Mercedes-Benz C280, 148,376 mi
had cracked vacuum block replaced but air only blows hot air
Visitor, 1995 Mercedes-Benz C280, 154,000 mi
STILL NOT WORKING
Visitor, 2007 Mercedes-Benz C280, 70,000 mi
Poor operation of the air circulation and temperature when turn to the lowest or highest degree
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).