EVAP leak codes — P0442 (small leak), P0455 (large leak), P0441 (incorrect purge flow), P0456 (very small leak) — are a check-engine light, not a running problem, and on a GTI they follow a short, cheap list. The EVAP system captures fuel-tank vapour in a charcoal canister and burns it through the purge valve; a leak or a stuck valve trips the code
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