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 S55 AMG Evaporator Temperature Sensor Failure
Mercedes-Benz S55 AMG Problem
Visitor, 2003 Mercedes-Benz S55 AMG, 78,000 mi
KLA V error 1298 which I think is rear evaporator sensor. A/C off light remains on even when pressing button. No cold ac. Reset and light turned off. Still need to replace sensor