Turbocharger Boost Control Problem - Golf4 AXR (Solution)

In the Golf 4 with an AXR engine, a leak in the vacuum control valve for the EGR cooling system can result in insufficient vacuum being available for the turbocharger pressure regulation.

"Check using the basic settings mode, e.g., with VAG-COM or VCDS. The difference between the "on" and "off" states of the actuator for turbocharger boost control should be approximately 150 mbar."
Otherwise, check the vacuum system for leaks, test the vacuum pump for proper function, and verify the functionality of the adjustment mechanism on the turbocharger.