The most common signs: the AC blows warm air even at full cold, you hear a loud grinding or squealing noise from the engine bay when the AC is on, the compressor pulley doesn't spin at all, or you notice oily residue near the compressor (a sign of a refrigerant leak). If you see any of these, WhatsApp us your car's brand, model and year — we'll tell you which compressor fits and whether it's worth repairing or replacing.