The evaporator temperature sensor can fail, causing the AC compressor not to cycle. If the AC compressor does not turn on, the AC system will not blow cold air. If the compressor is stuck on, the air will be very cold at first, before warming up.
Car Problem Reports
Mercedes-Benz C230 Evaporator Temperature Sensor Failure
Mercedes-Benz C230 Problem
The A/C would not cycle on.
After taking it to a shop they replaced a variety of pressure switches and other components, only to have the system fail two days later. I took it to a different mechanic and they quickly diagnosed the issue as a faulty temperature sensor - a relatively inexpensive fix.