A failed digital heater control valve can cause a lack of heat output from the climate control system.
Car Problem Reports
Mercedes-Benz E320 Failed Digital Heater Control Valve May Cause Poor Heat Output
Mercedes-Benz E320 Problem
Average Mileage: 69,951 mi (40,000 mi - 88,000 mi)
This is a small cheap part that costs about $100 to replace. Try this - under the hood, on the left side of the vehicle (as you look forwards) there's a plastic cover over the bulkhead behind the brake reservoir. Give it a healthy slap. Try your heater.
Visitor, 2005 Mercedes-Benz E320, 77,500 mi
Climate control blows cold air only no hot air
Visitor, 2004 Mercedes-Benz E320
My heater is blowing no heat just cool air...
Visitor, 2004 Mercedes-Benz E320, 63,255 mi
heater does not blow hot air only cold air
Visitor, 2004 Mercedes-Benz E320, 40,000 mi
had to change control system entirely.