A failed digital heater control valve can cause a lack of heat output from the climate control system.Google+
Car Problem Reports
Mercedes-Benz E350 4MATIC Failed Digital Heater Control Valve May Cause Poor Heat Output
Mercedes-Benz E350 4MATIC Problem
Average Mileage: 80,171 mi (9,375 mi - 114,000 mi)
Visitor, 2006 Mercedes-Benz E350 4MATIC, 62,000 mi
slow to produce heat in cabin, never gets what I would call hot.
Visitor, 2008 Mercedes-Benz E350 4MATIC, 100,000 mi
Temperature reading is around 50 deg celsius. It should be around 80 deg C. Car does not heat up in cold weather.
Visitor, 2010 Mercedes-Benz E350 4MATIC, 65,000 mi
No heat is coming out of the HVAC system, water temperature gauge is indicating <50 degrees celsius, should be around 80+
Visitor, 2013 Mercedes-Benz E350 4MATIC, 9,375 mi
Car not heating up except for when put in max