right, can anyone kindly tell me that, how can I test whether the duo value, or the temperature control of the ventilation system fail or not??

The reason I need help, because during the traffic jam in the sunny day, the air vent in the driver side will become very hot, just like you have turn on the heater,(just the driver side) but the setting is 18 degree C!!!

after we drive for about 3 min, the air vent cool down again. Is that high temperature come from the engine??


P.S: I don't have an air-condition in my 300E, as I am in UK.

It sounds like a vacuum system problem to me.

There isn't a lot of vacuum control in a European heater-only-system...
A lot of the W124s in Europe suffer from either a broken heater control or broken magnetic-switches. Either your control panel is broken (the part with the temp.setting wheels that is situated behind the center console panel) or the magnetic switch under the hood that opens/closes the warm water lines (left and right is controlled separate) that is situated just in front of the battery is broken.
My guess is the magnetic switch...

The center air vents blow cold air regardless the temperature settings you made.

greetingz,