P0299 — "turbocharger underboost" means the ECU asked for a boost pressure and the actual came up short by enough to log a fault. It's one of the most common codes I see on a turbo GTI, and it's almost never the turbo itself. I work it as a specified-vs-actual boost problem, cheap to expensive.
I read the data first. With VCDS Basics I log specified vs actual boost (and the wastegate/N75 duty) on a hard third-gear pull
…