A failed digital heater control valve can cause a lack of heat output from the climate control system.Google+
Car Problem Reports
Mercedes-Benz E55 AMG Failed Digital Heater Control Valve May Cause Poor Heat Output
Mercedes-Benz E55 AMG Problem
Average Mileage: 60,000 mi (50,000 mi - 70,000 mi)
Visitor, 2006 Mercedes-Benz E55 AMG, 50,000 mi
No heat output from climate control system. Replacing the Digital Heater Control Valve solved the problem. It turned out the the original valve was completely frozen, it could even be manually manipulated to open.
Over $ 600 cost to replace. Can air works fine but no heat......