Mercedes Benz Atego proportional valve

[medoice]

Hello,
I have some problems with Mercedes truck Atego year 2003. The ECU show me one error:
MR 7708 Proportional valve bank 1; short circuit to ground

The problem I noticed because when the truck idle.. suddenly just low the rpm to 400 and almost die, the I turn engine off, after 5 minutes runs without problems. But problem is that problem come again later.

Where is this valve located and what is used for ?
Thank you

[medoice]

The model is 1523 6 cylinder