AC blowing warm on a GTI is most often just low refrigerant from a slow leak — but diagnose in order.
Diagnose:
- Does the compressor clutch engage? If not, it's low refrigerant (a low-pressure lockout), the pressure switch, or wiring — not the compressor itself.
- Gauge it — low charge is the usual answer; recharge with UV dye to find the leak.
Common failures:
- A stone-holed condenser (it sits ahead of the radiator) — very common, and no recharge fixes a leak.
- Perished O-rings, or a failed compressor (noisy, seized clutch).
The fix: find and fix the leak first, replace the failed part (usually the condenser or O-rings), then evacuate and recharge to spec.
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