this happened to me before I sold my GP. the system is low on refrigerant as jorgs_7 said, also you do have a leak somewhere and should take it to a dealer. mine ended up needing a line or too. hopefully its not the condensor or its gonna be some higher dollars to fix.
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1973 Green Nova - SOLD
2000 Silver Grand Prix GT - SOLD
2001 Torch Red Monte Carlo SS - CURRENT