Non Return Valves (revisited)

[Ecxoes]

Hey guys
So now christmas and all that is out of the way, i got back on the over boost fault on the loon.
going through active tests with VCDS i was convinced i had found a fault with the solenoid for the diverter valves after it started clicking after i tapped it with a screw driver, so i replaced along with vac pipes etc,it but i still have a p1555 over boost fault.
I should add the n75 is also new
so my question is would a non return valve fault cause a overboost?
and if so what parts should be replaced and has anyone done this?

The boost fault is intermittent, sometimes it pulls like a train other times no boost, any ideas because I'm getting frustrated now.

cheers in advance