there can be a couple of possibilities on this
- mass air flow sensor not reading correctly causing low boost issue.
- n75 turbo waste gate contol solenoid .
- the turbo vanes inside the turbo is sticking causing overboost/underboost issues.
VAG com is needed to read the mass air flow sensor readings while driving to see if they are low . or a swap of maf sensors clearing the codes and road test to see if it comes back to life. (note if you have a 2.0 gas maf sensor to test will be just fine but it needs to be a 06a906461a part # )
need to start there the easiest and cheapest way to go .
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