I have a full stage 2 Astra Coupe, and I've had this exact same issue before where the car wouldn't overboost over 0.9bar - and in that instance I got the DV and BCS replaced and all was good again, the car was overboosting 1.2/1.3bar.
But once again the problem returned! So I recently replaced the DV with a new OEM unit and also swapped out the BCS with another test unit from a turbo that came off a friends car after he went the VXR upgrade route.
But still the car won't overboost, it only boosts 0.9bar at full throttle in 3rd and quickly drops off to about 0.4bar. This exact map use to over boost 1.2/1.3bar with ease.
Now I have had 2 different BCS's in the car to test which made no difference, could both of them be faulty units? Or should I be looking elsewhere, I have gone through the car for any vacuum leaks over and and over again and can't find any.
Could there be a communication problem between the actual BCS and the ECU maybe? I'm running out of ideas and really need to sort this issue!
Please anyone! Thanks in advance!