Lean-running codes — P0171 (system too lean), P2177/P2179 (lean off-idle), P2187/P2189 (lean at idle) — all say the same thing: the ECU is adding fuel to compensate for more air than it accounts for, usually unmetered air getting in after the MAF. On a turbo GTI the cause is almost always an air leak, the PCV, or the MAF — read the fuel trims and it tells you where.
Read fuel trims first with VCDS Basics
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