In the old days the turbo would boost as soon as the engine speed was sufficiently high and any over pressure would be dumped by a relief valve or controlled by a wastegate.
In these modern times of electronics the turbo wastegate is normally open instead of normally closed so there is no automatic boost. The engine management system decides when there will be boost and how much boost there will be.
Turbo chargers are amazingly reliable unless they are abused or there has been foreign matter ingress. When looking for the cause of no boost it is wise to disconnect the intake trunking and turn the compressor vanes by hand to ensure all is well.
Disconnect the vacuum line to the wastegate servo and apply suction to the servo to ensure the servo, the linkage and wastegate is moving and in good order.
The next stage is to examine the vacuum line and connections back to source and then ensure the vacuum supply is adequate.
If all is found to be well with those physical checks the cause of no boost is either the failure of one of the vacuum switches (or the electrical wiring or connections to them) or a fault condition exists that has caused the engine management to decide not to provide turbo boost.
In that case diagnostic equipment will be needed.
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