Ok this is just a possible fault that I found whilst investing a fault with my Turbo boost on 2008 Galaxy TDI resulting in a constant tripping into limp mode.

Please note I have included some pictures here to show a fault in the turbo boost valve "note" this not a serviceable component and if a fault does in fact exist with this valve it needs to be replaced.

I have cut this open to show the fault and operating parts so it can be fully understood regarding its operating function.

Note . a continuity check is first advised across the terminal pins to ensure the solenoids coil integrity...this is about 15 ohms.

The fault with the valves diaphragm is shown quite clearly and allow the incoming vacuum to leak to atmosphere so when the valve opened the signal did not reach the Turbo actuator diaphragm.

If this valve is functioning correctly then hose connections need to be checked as well as movement of the the VNT turbo operating linkage on the underside of the engine.

Hope this helps somebody... :-)

Thanks a lot for taking the time to tear that apart to allow both the likely fault to be observed and the explanation of the function!