Hi, welcome to the forum, sorry to hear you have a problem..
As far as I am aware a P0299 for lack of expected boost pressure fault has not been a common problem on the J VXR.
Could be a boost control or turbo problem, but referring to the drawing at,
http://ci.catcar.info/opel_2015_05/data/P10/G147B.png
first thing I would check for with the engine turned off is for is any signs of boost pressure air leaks (with a torch and a mirror on a stick maybe) from seal 15 between the turbine flange and pipe 14, that the two Torx screws 16 on the pipe flange are not loose, for any leaks in the hose part way along pipe 14, the connector between pipe 14 and pipe 18, pipe 18 and the connector to the intercooler 19, the intercooler itself, and the connector from the intercooler to pipe 20, pipe 20 itself, and charge pressure sensor 21 and check that this sensor is fitted correctly and the wiring looks good. Other than that, the next thing might be to record the boost pressure on a test drive with a diagnostic tool and check out the results to see what pressure you are actually getting at various engine speeds and under enthusiastic acceleration etc.
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